I'm assuming that you are looking to save some money if you are trying to buy for your horse or yourself on Ebay.  But what, exactly, do you need to have a well turned out horse?  Contrary to what many people would like you to believe, you do not need to own the top brands and styles.  Hunters/Equitation are classically conservative sports.  This allows you to buy good quality basics that will last a long time, instead of running out and getting the latest trend every year.  A quick run through of riders and horses attire for the hunters/equitation ring:

For the Rider:  Remember, fit is more important than than brand.  I suggest trying on a variety of brands at a local tack store to decide what you like for fit and feel.

Hunt coat
    Navy is the classic color, and always in style.  Grays are usually acceptable.  Generally, stay away from green and black.  Tans are in now, but could be out in a couple of years.
    Grand Prix, Tailored Sportsman, and RJ Classics are favorites.  Devon-Aire and Ovation make less expensive coats that are still good quality. 

Shirt
    White is the classic color for shirts, but most colors work.  As long as the color of your shirt looks good with your coat, it should be fine.  However, I suggest staying away from very loud colors. 
    For a very put together look, have your collar monogrammed.  This is a relatively cheap way to increase the style and elegance of your outfit. 
    NOTE: Stay away from stock pins!  These can come undone during a fall and puncture the throat.

Breeches
    Tan is always acceptable, and greenish beige is also a very popular color.  Do not buy white unless you are planning to ride in a Classic.
    Tailored Sportsman and Ariat are favorites for breeches, but many other brands are also well designed and good quality.  Devon-Aire, Tuff Rider, and Riding Sport are good quality alternatives.
    A note on Tailored Sportsman: These are very well made pants.  However, if you wear a full or wide tall boot, I suggest buying the four way stretch instead of the two way stretch.  There is a seam below the knee that can be tight over a fuller calf.

Field Boots:
    Field boots are black with lacing at the front of the ankle.
    Remember, fit is more important than brand.  This is especially true for tall boots.  At least three measurements should be used to determine boot sizing:  Foot size, widest part of the calf, and height from the back of the knee to the floor.  To assist fit, some boots now come with back zippers and elastic gussets.  Field boots should be skin tight over breeches.
    Favorite brands include Ariat and Vogel.  Other good quality, less expensive boots include Devon-Aire and Tuff Rider.  There are also various custom boot makers.  If you have not heard of them, ask to talk to customers of theirs so you can figure out the quality of the boot before buying.

Helmets:
     Black velvet is the classic choice, but GPAs have become very popular as well.
    Different helmets fit in different ways.  With helmets, it is absolutely necessary to choose fit over style.  I do not suggest buying a used helmet, as there is no way to know if the helmet is still safe.  Make sure the helmet you choose is ASTM/SEI certified (a safety rating.)
    Popular brands include Charles Owen (velvet) and GPA (synthetic).  Other velvet helmets include International and Troxel.  An alternative to the GPA is International's ATH line. (While the top of the line ATHs can be just as expensive as GPAs, they have a model that retails for around $100.)

Other Rider Attire:
    Hairnets are used to neatly tuck the hair under the helmet.  Some people argue that this makes the fit of the helmet ineffective, but I don't believe there have been any studies to prove this.  An much less popular alternative is a show bow.  Some people consider show bows untidy and unattractive.
    Gloves should be black leather.  SSG is popular, but any thin black leather glove should work.  If you have VERY quiet hands, you may consider lighter colored gloves.  However, the vast majority of people should wear black gloves.
    Belts should be dark and conservative.  If your breeches zip in front, the belt buckle is centered in front.  If they zip on the side, the belt is buckled on that side.
    Tall socks help slide boots on.  They are cheap and the on part of hunter/jumper attire that can be as loud as you want.  They should never show above the top of your boot.


For the Horse:  Again, fit is more important than than brand.  If you can find anything at a local tack store or owned by a friend, feel of the leather (or other material) and try it on your horse if possible.

Saddle:
    Saddles should be some shade of brown, NOT black.  As a rule, dark horses should wear darker leather.
    Saddles come in numerous styles and fits.  It is necessary to try a saddle for fit before buying, unless you feel comfortable reselling.
    Popular brands include Beval, Pessoa, Tad Coffin, PJs, Jaguar and Excelle.  Hermes and Antares are beautiful saddles, but very expensive.  Good quality but less expensive saddles include Crosby, Stubben, HDR, Collegiate and Dover's Circuit line.  The best bet for getting a good quality saddle for less is to buy used.

Bridle (and Martingale):
    Should match your saddle, colorwise.
    Padded nosebands and browbands, fancy stitching, and raised leather are ways to make the bridle fancier.  Whichever you choose, if you choose any at all, is a matter of preference.
    Brands typically match saddle brands.    However, there are also Wellington, Jimmy's 21st Century, and Edgewood bridles, all very popular.  Other brands include Wylde and Vespucci.  Less expensive bridles include Suffolk and Ovation.
    NOTE: Many bridles are sold without reins.

Pad:
    Must be white and show 1" of fleece uniformly around the saddle.  Can be made of synthetic materials or sheepskin.
    Brand does not seem to have much effect on quality of synthetic pads.  For fleece pads, either choose one you have felt or one you think you can resell if you need to.  Fleeceworks and Equine Comfort make good quality sheepskin pads.

Girth:
    Should either match or complement the color of your saddle.  Follow the dark horse/dark leather rule.  Many saddle brands have matching girths.
    Girths can be contured to fit the horse better, fancy stitched, have fleece lining, a belly guard, and/or elastic inserts.  While this is all personal choice, I highly suggest choosing a girth with elastic in it.  Elastic helps prevent overtightening, which has been proven to reduce performance.
    Some popular brands include Tad Coffin, Jimmy's 21st Century, Passier, Jaguar, BDH, Beval, and Pessoa.  Dover, Collegiate, Crosby, Stubben, HDR and Ovation also make nice girths. 

Leg Protection:  Note that leg protection can only be used in the Equitation or Jumper rings. 
    Bell Boots should match hooves: black on black, or gum on light.   Pull on styles look more put together than velcro closure.
    Open Front Jumping Boots:
    Leather boots should be brown and complement your saddle.   Popular brands include Beval and Pessoa.  Dover and Wylde also make good quality boots.  They can be lined in fleece, rubber of leather.
    Synthetic boots should be brown or black on very dark horses (or white horses.)  Eskadrons are by far the most popular.
    Ankle boots should match front boots.
    Polo wraps can also be used.  These should be black.  If your horse has white stockings you may use white.

Horse Attire at the Stable:
    This gets even hazier as to what looks great and what doesn't.  It is much more a matter of opinion than show gear.  So I'm just going to briefly touch on brands that I know to be of good quality in some basic stable gear.

Blankets:
Rambo has very high quality, but expensive, turnouts, sheets, and fly sheets
Weatherbeeta as well, but they also make nice coolers, and are a bit cheaper
Rider's Horse Clothing International did not impress me with quality, but are inexpensive and will replace any damaged blankets.  However, I would not suggest buying one over Ebay, because you lose the ability to return it.
Cashel makes excellent, high quality fly masks
Blue Ribbon and Triple Crown are custom blankets, great quality.  Triple Crown is the better known of the two.
Baker makes wonderful stable rugs and sheets, as well as other blankets now.  The Baker plaid is quite iconic.
Pessoa makes good quality turnouts, sheets, and coolers.
Saratoga makes less expensive, good quality blankets, sheets, and coolers.  Some Custom

Also:
A leather halter (add a nameplate for around $10, and it makes the halter look a lot nicer)
A leather lead with chain- even if you never intend to use the chain, it can just rest against the horses skin.  Chain leads were orginially for safety, and are now 'the look' at shows.