Dr. Bill Dorfman -- a world famous dentist who treats some of the biggest stars on Earth -- tells TMZ he keeps a "secret stash" of the teeth he's plucked from the mouths of his famous patients ... in the hopes of eventually selling them on eBay.

Dorfman -- who recently treated Lindsay Lohan and whose website features Jessica Simpson, Usher, Ozzy Osbourne, Anne Hathaway and Eva Longoria -- was outside Rock & Brews in El Segundo yesterday when he revealed his secret.

"I actually save famous people's teeth when I pull them ... but I can't tell you [their identities] 'cause it's like patient confidentiality."

He continues, "There have been a few really famous people and I thought one day maybe I could sell this on eBay."

Dorfman insists he always offers the teeth to his patients first ... but they often decline, "So I put it in the back and i just hold on to it."

One dentist told us, "What Dorfman is doing is just sick."