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.