The first thing to keep in mind when you are thinking of buying a signed book on eBay is that unfortunately there are a good many unscrupulous, or just plain unaware people selling 'signed' or inscribed books. Some may be only tiny in price, some could cost you thousands.
In my experience as both buyer, collector and seller of signed books is that some authors are more frequently forged than many others, this number normally matches their popularity as collector's items. Harry Potter, Stephen King, C. S. Lewis, J.R.R. Tolkien, Stephanie Meyer, Ernest Hemingway, A.A. Milne, Enid Blyton. These are just a few of the well-known authors that are very frequently forged and sold to unaware buyers on eBay.
Points to be very aware of when you are viewing the item are things such as:
1. Does the seller have good feedback?
2. What other items have they been selling or buying? Review their feedback thoroughly, for both given and received feedback.
3. Read their description very carefully.
4. If they say anything like "This appears genuine", or, "I don't have any other information about where it was signed" or anything similar, then avoid the listing and move on!
5. Be very wary of sellers who give a hugely lengthy story about how they came to obtain a signed copy of say Harry Potter and The Half Blood Prince. For example, they know someone at the publisher who met J.K. Rowling and had it signed blah blah, these kinds of events are unlikely in general.
6. Look and compare! Review their photos very carefully, seek out other examples of the author's signature on google. Look carefully at the shape, size and style of the signature on a known genuine example and compare it closely to the eBay item you are considering purchasing. Look at distinctive patterns in the signature. If they are not similar and you get a feeling it may be not genuine then move on and do not bid.
7. Ask the seller to provide details about the authenticity or history of the item. Any genuine person will reply with a straightforward response, not a long drawn out story which distracts from the item in question!
8. Look at the price. If somebody has posted an item such as a 'signed' copy of The Lion, The Witch and The Wardrobe by C.S. Lewis with a 0.99 start price and a blurred photo, alongside any of the above issues, then move on!
9. If in doubt at all with any of the points I raise, move on!
10. Be patient, hold on to your money and wait for someone to sell a genuine, honest signed copy of the book you are after.
Good luck on eBay. Buying signed books or autographed items generally is a minefield. Stick to proper sellers, preferably business sellers and always use instinct, never rush into buying anything without taking a little time to do some research first people!
Be annoying and ask questions, do not be too polite to bug people - honest sellers will welcome questions and queries!