Booksellers have a pretty clear definition of just what is “good” condition for a book. If the books are offered by people who are NOT booksellers, you might want to ask them just what they mean by good. I believe that you’ll need an eBay account and be logged in to use the little button that says “Ask the seller a question”, but I’ve used this button several times and had good luck with it.
Do you want this book for collection purposes, or do you just want a reading copy? Do you care whether the dust jacket (if it’s a hardbound book) is in near perfect condition, or even present? Only you can decide what is acceptable for your purposes, and how much you’re willing to pay to get it. A book in mint condition, with a perfect dust jacket, is likely to cost MUCH more than a book with a broken spine, dogeared and “tanned” pages, and no dust jacket.
I agree with everyone else. Read the feedback! If there’s only positive feedback, I just skim through it. If the seller has hundreds or thousands of feedbacks, I’m willing to overlook a few neutral or even negative complaints, but I DO look at what the complaints are about. Some buyers are just not happy with anything, so I discount their complaints. But if a seller has too many negative feedbacks, I will not buy from that person. I’ve found that sooner or later, just about EVERYTHING that I could want and afford will show up on eBay, so I do not have to bid on an item at a particular time.
I’ve had good results buying used books from both eBay and Amazon’s used book division. My local used book store (and, since she works there, now my daughter) can also frequently find a book for me online, through the various online used book dealers. There’s Bibliofind and ABE, for instance. Sorry, I don’t have links to them, but I’m sure that others do.
You can also try just using Google to find mention of the books you want. This will frequently bring up places that have it for sale. I’ve done this a time or two, hunting for books that I remember from childhood.
Good luck, and I hope you get what you’re looking for.