I've put some characters for sale on the Onyxia character trade system, however there is no way to see which mounts they have. On other servers you can see the mount list.

My suggestion is to make the mount list public, even if just for characters on sale, because if a character has a 100% mount it is very important for the buyer to know. A fast mount costs around 1k gold which is around 20 coins, so it is very impactful. As it stands, the buyer just have to gamble it seems