I buy most of my horns on eBay and have learned, through long and expensive experience, to examine every instrument with a fine toothed comb before purchasing! I've received cracked clarinets, plastic instead of wood clarinets, broken clarinets, clarinets with damaged or missing keys or rings, and even a clarinet that was made of pieces of 3 different clarinets! So, based on my experience and some of the customer horns I've been seeing lately (also purchased on eBay) I'd say its time for an eBay buying tutorial!
First, I'd restrict my buying to the US and Canada unless you are fluent in another language and can afford to risk the high shipping. Mistakes can happen with overseas communication, and it can be much harder and more expensive to force a return if things aren't as represented!
Second, look at feedback: while feedback can't tell you everything (with one negative, ebay's current policy of basing it on only the last year's transactions can make a small seller look worse than he/she is) it is a good place to start. Numerous negatives/ neutrals and unflattering comments about misrepresented condition, poor communication, and high shipping can be a tip off that you aren't dealing with someone who is professional or honest - best to move on; lots of fish in the sea.
Third, what does the ad show and tell you? Is it full of sharp pictures and copious details or a few badly taken pictures and a very skimpy description posted from a mobile phone? Is the person obviously knowledgeable and informative about the instrument, or just a garage sale picker looking for a quick sale at the highest price?
Fourth, is there a return policy? Honest, knowledgeable sellers know that mistakes can happen, especially if they are selling something that they are not familiar with - I had to return a clarinet just a few weeks ago, as the ad failed to show or mention that the upper half of the bridge key was broken off and missing! Luckily, she was honest and pleasant about it, but I haven't always been so lucky...
To be continued!