Some auctions will be tied to a state-level Surrender program, in association with Habitattitude. The purpose is to allow anyone with a fish that is not wanted an outlet to "surrender" the fish so that MAS can find it a new home. (...not wanted for any reason including too big or too aggressive.) The aquarium world agrees that this is the responsible approach as opposed to releasing captive fish into the local ponds or streams.
Here is how it works...
If the auction is designated as a "Surrender" event, just bring the fish to the auction (preferrable in a bag, but all water-holding containers such as a bucket are acceptable). The fish should be dropped off within an hour of the posted start time for the auction.
That is all you need to do. Being that a hundred or so aquarists will be at this event, the fish will first be offered publically to anyone who wants it. If for whatever reason the fish cannot be given away, MAS will responsibly handle the fish.
See this article discussing the the first Surrender event.