Just my experience, as a buyer with over 300 eBay transactions under my belt:
Good pictures are worth more than a thousand words, especially from someone who is being a "salesman." (No offense to the sales professionals with integrity.) You know the type..."this bike was only ridden by a little old lady who rode it to church on Sundays." Or someone claiming a PX-10 is the best bike ever made, etc. If you sense any overstatement or fluffing, stay away.
Feedback on eBay is like location in Real Estate.
Ask pertinent questions and only bid if you get satisfactory answers. But, as Richard stated, don't expect the seller to be an expert. Some of the best deals I've gotten on eBay were because the item wasn't listed properly. Campagnola rules!
Don't buy from anyone who states in their listings: Absolutely no returns or as is, final sale, etc. Wonder why the person is so worried you aren't going to be satisfied.
If the sale goes sour, stay objective and polite, but firm. I've been sure I was taken on a purchase, then it turned out it was just a communication problem. Other times, I was able to negotiate a full or partial refund. And yes, occassionally things don't go as well as you'd hoped they would. The deals and savings far outweigh the ones you just have to write off. Just like in baseball and life, you can't hit a home run every time.
Trust your intuition.
Good Luck and happy hunting.. Just don't bid against doghouse44 ;-)
Ed Kasper
Detroit