On Ebay you sometimes come up with situations where you are truely dissatisfied with the results of your purchase.  What to do?

1) First, go back and read carefully the advertisment for the product.  Did you read it carefully before purchasing and was the advertisment accurate?  If you failed to read the advertisment and 'missed' some critical piece of information, learn from your mistake and go on. 

2) Read the seller's policies regarding feedback.  Many sellers automatically issue negative feedback if you do so to them.  In other words, they hold a gun to your head and threaten you before you have said a word about your experience with them.  Yes, you can request the negative feedback be removed from your account, but isn't it better to avoid those sellers who will automatically retaliate in the first place?

3) Give the seller a reasonable chance.  Once I puchased a part from a seller who didn't test the part completely.  The part was sent "working" but when tested it was not fully functional.  In the advertisment the seller said the part was "fully functional" and for the most part it was -- unfortunately it failed the one crucial area where it was needed for the project we had in mind.  I contacted the seller and explained the problem -- to which he replied that the item was sold "as is."  The problem was, he represented the "is" as "fully functional."  Obviously his advertisment wasn't accurate as there was some confusion about what he meant by "working" and what I expected.  So we compromised and split the purchase price.  I learned to take "as is" to mean "it may not work at all" dispite what the rest of the ad said, and he, I believe, learned "fully functional" should not be used as a description if you haven't checked the functionality fully.

In any case, give the seller a chance.  And when you do don't begin by assumming the worst.  Assume that the whole thing is your fault from the beginning and let the seller show you how you misread the ad, didn't understand the terms, or just didn't know what questions to ask.  This doesn't mean you roll over but it does mean if you have contributed to the problem in some small way and are willing to admit it, that the seller, if her or she is an 'honest bloke' will be more willing to admit where they fell short.  In the end it means that you may not receive 100% refund and payment of return shipping, but isn't 80% and no negative feedback better than fighting with the seller and both of you giving negative feedback?

4) When in doubt eat it.  If you have any doubt as to your position -- even the slightest -- understand that the seller will see that area as the "crux" of the problem.  In other words while your minimize your mistake(s) he or she will be busy minimizing his or her errors and maximizing yours.  So put yourself in their shoes and ask if you were in business just to make a buck if you would care enough to do much about the situation.  After all, the seller has the money, they have a 98.5% positive feedback rating (doesn't 'holding a gun' to the head of the buyer do wonders?), and all they have to do is find the slimmest of errors on your part to justify (to themselves at least) their position.  You are not, as the buyer, likely to be holding a strong position.  And since you probably purchase far fewer items than you sell a single negative feedback rating may effect you more than one more negative feedback rating effects them.  So you might as well eat it.

5) If you are certain that the seller is completely and utterly in the wrong and you have (politely but firmly) done all you can to get them to do the right thing, don't bother leaving negative feedback -- it only hurts you -- but, instead get the opinion of a few others -- preferably indepent people who know the situation on Ebay -- and see if they agree.  I once had a problem with something and went to the forum to ask if I was in the wrong.  Almost everybody said I was, so I "shut up already!"

6) Finally, if everybody agrees you were completely and utterly in the right, and if you have good documentation of everything said and done in the transaction, do the formal complaint route and see if you can get your money refunded via Ebay's resolution process.  Again, though, don't bother leaving feedback.  At least that's my opinion.

Hopefully, if you follow the steps above you will never get to steps 4, 5, or (heaven forbid) 6!  And you will be happy with the results.