Depends on the rally....it's more likely at the larger regional ones.
We were at the Balloon Fiesta Rally (+/-250 A/S's) in Albruquerque, NM in October and there were several trailers bought sold and traded! A couple of A/S dealers brought new trailers to sell and folks traded in their oldies and went home with new ones. The traded in older ones were then offered for sale too...in the case of the vintage ones, they were mostly for sale by owner.
I know of four trailers that traded hands within our group alone. One of which was a vintage, two were only 1 year or two old and the other was a late 70's.
It was like a big game of musical chairs...everybody cleaning out their old stuff and packing up the new ones.
In my opinion, one of the best ways to find a trailer is through WBCCI folks...someone always knows someone who has one for sale!
Good luck in your search ~