I have sold dozens of trumpets on eBay. I've bought a couple on eBay. Seller is held to the description ( I know I am ). I play all that I sell and advertise that. Play it straight and you'll never get a return. Go the extra mile and get great feedback. We are a 100% positive feedback super seller because we don't sell junk. There are a few eBay sellers that I just laught at. They are always over priced and stiff the buyer somehow. If you offer a return policy then you get sales. Stay away from the no return policy sellers.
If the ad states that it plays well....good tone..fast valves...etc.....you can hold them to that with a block on their PayPal account if it has sticking valves or a hole in the thing or something. Yes certainly ask questions thru their eBay site. They are held financially responsible to their answers as well. The best places to find deals on trumpets are yard sales...flea markets...and thrift stores where you can examine and play them on the spot.