You may basically have to give many of those larger fish away, because most experienced aquarists don't have room for them, and if they go to a LFS, they may sit, and need to be fed for months before someone has room for them, becoming a losing proposition for the store.
If you have a local aquarium club, you could take them to one of the bi-annual auctions, or regular swap meets.
If someone is looking for a certain type they may be bid it up, and you may get e reasonable price at a swap, but I often see large cichlids, and other fish go for a $1 or $2 each.
If you want Vieja, I believe a 125 is only large enough for a pair, and agree that tank is also "too smal"l for an arowana, or stingray.