Where can you sell turquoise?

It depends upon exactly what you're selling. If you run a turquoise mine, you sell to overseas cutting shops or skilled domestic lapidaries. If you mean a single piece of turquoise jewelry, try Craigslist or your friends. If you're proposing to sell a line of turquoise jewelry, build a website, get a booth at local gem/jewelry shows, hit the shows at Tucson, or talk to your local jewelry store.

A more specific question will receive a more specific answer.