Do you buy watches new or 2nd hand?
I'm on TZ Forum, and that's a great place for watch flippers (instead of selling or buying from dealers, you deal directly with folks like you or me). Could do direct trades, swapsies...
Iconic Watches are excellent grey dealers - I've bought from them before and their preowned pieces are fairly priced. Part exchange rates are shit but nowadays i never buy new so I rarely see a loss.
If you do use EBay, it's best to meet folk face to face, pref in the city/ in public, I sold my Omega AT outside of the dealership store...
What have you in mind?
ETA - My friend started a watch reselling business but he only deals with Rolexes, Breitlings, Omega, Pams...let me know if you want his details.