Does this store trade fishes of equal value

Have no idea my friend just brought a fish tank best to call store and ask

Nope

No

As someone that ran petstores, that would not make economic sense. The fish they have has been acclimated to the stores water and is presumably healthy. Your trade in could be old or your water quality could be vastly different from the stores, and your fish could die. I used to give 50% store credit, and only for fish in immaculate condition. Stores are in the business of making money, not trading.