Yes, Petco will buy your fish. If you have a fish that is common and in good health, Petco will likely give you a store credit that can be used to purchase other fish or supplies.
The credit is usually for slightly less than the retail value of the fish. less-common or unhealthy fish may be bought for a smaller amount or not at all.
To get the most value for your fish, bring them to Petco when they are healthy and common. Unhealthy or less common fish may not be bought at all, or may only be bought for a fraction of the retail value.
Store credit is usually given for healthy, common fish.