We have a local used tack sale.
The tack store itself runs it. It takes items on commision, then put them out by category at the sale. Anything that sells, they take 25%, and pay the owner of the tack the remainder. I don't know how much they advertise, but they do send out cards to all their regular customers, and I'm sure I've seen their ad for it in a horse magazine once or twice!
Ask if you have any more questions.