This brief guide assumes basic photo knowledge & covers 5 specific topics:

1. Filters for film vs. digital
2. Cokin vs. traditional glass filters
3. Top filter brands
4. Things to watch for when buying filters on eBay
5. Interpreting used equipment ratings

The most useful filters for digital are:

• UV to protect lenses. Sensors aren’t UV sensitive, so you’re just looking for the protection. eBay’s guide incorrectly lumps Skylight filters into this category, but they have a slightly warm color, which should be avoided.
• Polarizer to control reflections, sky density, specular highlights & color saturation. eBay says that auto-focus cameras call for circular polarizing filters, but that’s a misleading generalization. It depends on the type of auto-focus your camera uses. AF SLR’s / DSLR's use beam-splitting mirrors that require circular. Point & shoot cameras, including consumer digital models, work equally well with linear or circular polarizing filters. I have another eBay guide dedicated specifically to polarizers: Circular-vs-Linear-Polarizing-Filters
• Special Effects that would be difficult to accomplish w/ software. This includes cross-star, multi-image & close-up lenses. ND could be in this category, for long daylight exposures (may also be needed if your camera can’t adjust to extremely bright light).
• Graduated filters are a bit of a judgment call. Processing & blending different versions of the same RAW image file works much better, but using a filter might be easier. I carry a gray grad myself.
• Soft-effects are another maybe. While I prefer to avoid any sharpening, for a slightly soft look, other photographers say that a filter is the only way to get the effect they want. A sophisticated filter like a Softar, layers a sharp image with a slightly diffused one. Blending sharp & unsharp layers in Photoshop doesn’t give the same result.
• Filters intended for B&W film don’t work w/ digital, with the exception of Infrared. However, not all camera models work well for infrared (because there’s an IR blocking filter over the sensor). Try a Yahoo or Google search for infrared + your camera model to see if people are experiencing success. You can also search for samples of infrared photos, if you’re not familiar with their distinctive look. Note that inexpensive 25 & 29 red filters used for b&w film don’t work for digital. You’ll need the more expensive RM72 (the even more expensive RM90 is popular with people who think they’re going to photograph through clothing, but it’s not a very worthwhile trick).
• Color conversion filters have no digital application because the color of a digital image is determined by software.

Top Filter Brands

Nikon, Minolta, B+W, Heliopan, Hoya HMC & Tiffen TMC command a premium. While B+W is highly regarded, Nikon is my personal favorite. Minolta is a “sleeper” brand, nearly as good w/o the high demand (but harder to find). The Hoya’s made in Japan have great glass, although retaining rings sometimes pop out. The finish on a Tiffen ring is a little less inspired, but they stay together. As for filters made in developing countries, you might do just as well making them yourself at home.

* see footnote regarding Nikon polarizers

Cokin Filters

• These are cheap, fragile, plastic filters that can be tedious to use, but are quite popular & offer some unique advantages.
• One Cokin advantage is that special-effect filters can be flexibly positioned for a desired result.
• Another  BIG advantage is that the holder is designed for multiple filters, which can be positioned independently. For example, if you try to align a standard threaded polarizer & graduated ND filter, you might eventually give up & go home. But, this can easily be accomplished w/ Cokin.
• A set of Cokin filters can fit a variety of lens sizes.

Buying Filters on eBay

• Condition is the main issue:
• Few sellers clean** their filters, which is required to assess condition.
• Older filters that “need cleaning” are likely to have some fungus damage to the coating.
• Look for filter cases. Filters in cases are likely to be in better condition.

** I have another eBay guide dedicated specifically to cleaning: Cleaning-Photographic-Lenses-and-Filters

• A 50/50 proposition: eBay says, “buy with confidence,” but if you are discerning about your equipment, prepare to be a little disappointed, & bid accordingly. A money-back guarantee isn’t worth much when round-trip shipping is equal to 50% of your cost. However, retail markup on filters is high, so eBay is still the best place to get a deal.
• Adjective inflation: according to photo-dealer ratings in common use “excellent” is average, & “good” is below average. Familiarize yourself w/ dealer ratings below, then check out a seller’s other auctions to determine what “language” they’re using.

Typical Dealer Ratings

• NEW:  Never Used - All original packaging and manuals included.
• MINT:  Like New with little or no signs of use. 98-100% of original condition.
• MINT- (mint minus):  May have slight wear but only visible under close up inspection - 96-97% of original condition.
• EXC+ (excellent plus):  90-95% of original condition - Lens Glass very clean - cosmetically may show slight wear and/or signs of use. 
• EXC (Excellent):  85-89% of original condition. Shows signs of moderate use - Lens Glass is perfect but may have some dust which will not affect picture quality.
• GOOD:  Appears well used and may include dings, brassing, scrapes and bruises but is in fully functional condition. 75-84% of original condition. Glass may have cleaning marks or small scratches which won't affect image quality.
• FAIR:  Appears to have been used very heavily with multiple dings, scrapes, scratches, and heavy brassing all possible. Lens Glass may have fungus, excessive dust and/or scratches that will likely affect picture quality.

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* Footnote: I've seen so many defective Nikon Polarizers, that I've decided to add a disclaimer to my preference for Nikon filters. High-end filter makers (including Nikon, B+W, Heliopan & Hoya) use lot-dyed optical glass for most of their product line. Tiffen, & most lower-end manufacturers, sandwich a colored gel layer between clear glass (in developing countries it’s often just colored glue). However, Polarizers are always sandwiched (glass + 1 layer for linear, or glass + 2 layers for Circular). Tiffen has been making sandwiched filters in high quantities for a long time, & they are very good at it; Nikon apparently is not. About 75% of the older Nikon PL's I've seen have internal delamination, even when the filters are in otherwise mint condition. While it's not an obvious visual flaw, it does noticeably soften photos. This footnote applies mainly to used filters because they delaminate over a long period of time. However, based on a sample of 100 new B+W F-Pro circular PL's, another manufacturer that specializes in lot-dyed optical glass, about 5% had subtle lamination issues (not wrinkles like Nikon, just a slight haze or texture that probably wouldn't affect photos). Hoya & Tiffen delamination is also fairly uncommon, although there can be other issues.

It seems that competitors like to vote against my guides & I expect some disagreement from Nikon fans, so please consider that I closely inspect 1,000's of filters a year, & sell all of these brands. The Nikon Polar filter pictured below appears to be in perfect LN condition, but the lamination flaw shows up when a very bright miniature halogen lamp is reflected in the surface. It’s difficult to photograph because only a few lamination wrinkles can be seen at a time. right next to the reflection, but they cover the entire filter area. These small photos are the maximum size allowed in guides.