I kind of enjoy when people post signatures.I like to see if I guess wrong or right as to whether they are legit.I agree that buying from reputable known sellers can ease your worries but it can be fun to troll ebay for them just like trolling Goodwill,ect. There ARE some good deals to be had on occasion.I DID dole out $100 for a signed BCE of The Dead Zone once as it seemed a good deal but then I hemmed and hawed over the signature for like 8 months before asking some folks who seem to know their stuff what they thought of it.The concensus was that it was legit so I was a happy camper!
There are a ton of obvious (to us) fake Stephen King signed books on eBay, lots that look good at a quick glance, but are forgeries, and even a few sellers that are actually getting good at forging Steve's signature. A clue is to look at the sellers other items, including their sold items. If seller X has sold 50-60 signed King books in the last few months, question how did they get that many? Does the seller always have tons of signed books for sale? If the seller is located in East Timbuktu, ask yourself how did a person in that remote location get so many signed books in the first place? Do the Stephen King sigs resemble the JK Rowling, Anne Rice and every other signed book they are selling?
If the key to real estate is location, location, location, the key to an authentic signed book is provenance, provenance, provenance!.
Where was the book signed? At an event? Where and when? (Did such an event actually happen?) Older books that are flat signed back when Steve only inscribed books? Could be a fake.