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. Ebay will sometimes assist you in forcing a return if an item has been misrepresented, but it is a troublesome process and they keep track of how often you use it: too many returns, and you risk losing access to the service, whether you are in the wrong or not!
So, fifth and last - ask questions! Here are some of my favorites:
'Is the instrument in playable condition?'
'Are all pieces wood?'
'Do all pieces have logos that match?'
'Are there any cracks, chips, repairs, or any missing, damaged or frozen keys?'
'What is your return policy?'
If you get what seem like thoughtful, honest answers, their feedback is in an acceptable range, and the photos look good, you are most likely safe to go ahead. If you receive no answer or something like "I know nothing about clarinets but it looks good to me" (one of my favorites) then be prepared to pass, or set a bid at a low price, or ask more questions.
Also, avoid tunnel vision or 'gotta have it' syndrome: there are lots of clarinets for sale this week and there will be next week, too. If you're not a 100% percent satisfied with what you are looking at, look some more. Remember, its just a clarinet....