Ebay, or eBay, is one of my most visited sites on my computers.  It is like window shopping without the waste of fuel, it is one of the best resources to find out about a product, and it is even entertaining.  But, when I have a little extra cash, eBay is my first choice to shop.
I look through the items that I am searching for.  I do change the postings to show lower prices first and then work my way up going by my budget.  If I am looking for a comic book and I don't want to spend more than $10 than I am also going to pay attention to the shipping charges as well. 
When a product has hit my eye, it is time to do some research.  I read everything the buyer has to say about the product.  If it is a book, are there going to be marks or notes in it, is the cover worn away or damaged, and sometimes I even look for the edition number of the book.  If I were looking to buy an updated encyclopedia set I am not going to buy early editions unless it is something like 2007 or 2008 brand new set.  If I want to buy old encyclopedias, then yes the earlier edition the better.
Many things also can have the same title.  If I am looking for a particular movie, it is amazing how many remakes there can be for more than one movie.  I look to make sure it is the one I am looking for.  I do want to buy a newer version of something like "Psycho" is I want the old black and white one with the true Norman Bates in it.
Once I have decided "yep, this is the one" I continue my reading.  I look to see how many people have bid on it.  Sometimes, if I see there is a tug of war going on with a few people, normally I would let them have at it and go on to find something else.  Otherwise, I will put my bid in and see if the war continues.  Perhaps, I will add a higher amount but it is really going to depend on me asking "how much do I really want this product".  If I really, really want it then I am going to join the war.
Misspelled words in an ad sometimes bothers me as the seller didn't take the time to correct errors.  If there are quite a few misspellings then I move on to something else.  It only shows me that if there is not careful consideration in their verbiage and time to correct errors, how will I know that I am going to get a product that has been carefully wrapped and shipped or what I thought the item should have been to begin with.
Looking at the seller's star ratings is very important.  Very rarely will I purchase from a new seller simply because there is not enough known about this buyer.  But, then again it will depend upon how badly I want the product. If they are a power seller then I read their percentages of positive vs. negative.  I do give a few the shadow of the doubt when I read the negative feedback came from someone who didn't like the colour or some similar reason for not being satisified with the purchase,  And if someone has sold over 100 items and there is one or two neutral or negative remarks, I am not going to worry about my purchase.  You can't please everyone all of the time.  But, the seller hopefully will either offer a refund or a replacement.
Communicating with the seller is good, too.  Asking questions can tell you if it is a light purple or a dark purple or if its ever been worn before or if it has zippers or buttons, or whatever your concerns are.  I absolutely love the sellers that answer right back and more so when they actually greet me first before they answer the question.  Great communication goes a long way with me.
Be careful with grab bags.  They do state in most of them that the box you are purchasing is simply a box and really they don't have to send you anything more than a box.  Which I don't need any boxes, so I move on.  Some of them however will tell you that the items in the box will be of similiar things to such and such.  Some of them will also show that part of your bid will go to a charity of some sort.  I personally have not had any luck with this type of purchase.  The last one I did, I got a box of stuff that I didn't know what to do with any of it and some of it was broken.  And I believe that whatever charitable orgazination that seller used only put ten cents of my bid to charity. But, that is not to say there aren't good grab bags out there and some people are quite happy with their purchases when they do get their box.  So, let your gut be your guide.
Okay, to break it all down:  Read the complete description of product, read the shipping options, know how much you are spending by adding your bid price and shipping costs, ask the seller questions when in doubt, fight when you thinks it is something to be fought over.  But, ABOVE ALL ELSE, stay within your budget and don't go crazy when you only have $50 to spend and you have bid on things that have gone beyond your cash flow.  In the early days of eBay, I had literally spent weekly income to pay for my won purchases, I had become addicted.  So, be safe and be smart in your shopping.
Good luck and happy eBaying!!!!