I have fish shipped to me all the time. Use enough water and six hours in a bag should not be horribly stressful on the fish. Use judgment on what you bring(don't bring a "reef-safe" angler in an 8 ounce bag of water) and the fish should have no problem surviving(my philosophy: if it doesn't survive a quick and safe transport, it probably wouldn't survive at all).
There is no rule against bringing inverts and fish and anyone is welcome to do so. If you are bringing a fish, you should give the curtousey of explaining why you don't want it anymore, i.e. it outgrew my tank, chased this fish, brought ich into the system(J/K)...being honest is important when someone else is trusting you to be auctioning a good product. Maybe also mention minimum tank requirements, tank size, feeding patterns, what you've fed, how long he's been in your tank.