There are many reasons why lfs do not offer cash for fish and only give credit towards other fish. Much has to do with the cost the store can purchase the fish from compared to what you think you should get for your fish. The mark-up on fish is significantly different than it is on dry goods. As for the fish you are talking about, there is simply not enough demand for most lfs to house your 40 red devils or dovi. They will quickly grow and will require seperation in order to ensure good health and minimal body damage.
Credit comes with its own set of issues for a store. Customers horde it until they have hundred of dollars accumulated and then attempt to use it all at once. Sounds great for the customer, but that negatively impacts the stores gains for the day. A $150 fish sale is far different from a $150 fish sale using credit. Customers lose their credit slips and expect the store to keep a record or "remember" what it was they traded and the value in credit that was given. Customer's credit falls short of what it is they want and so they haggle to try to get the fish they wanted. Does that work at Petsmart? Mom and Pop shops depend on every dime they get. Temporary financial loss for a M&P can be the difference of going out of business (or appearing that way for significant amounts of time).
The bottom line is that stores want your business. Honestly, we NEED it. So we try to work with you to keep your business and help you out. If when you purchased the fish you did not get it from us, we are doing you the favor of helping you out. The same goes for the person looking to unload large amounts of fish when they do not own a business license and the lfs had made no prior agreement to use you as a breeder.
For those of you looking to get credit for your fish, keep in mind the following:
1. Having a long standing, good relationship with the owner/manager helps you get more in credit.
2. Shopping with that store on a regular basis (even just for dechlor and food) increases the chances that the store will work with you.
3. Frequent the store and notice what they have sold out of or have not had in a while. If you have what we want, we will work something out to get it.
4. Smaller amounts on a regular basis help a store out much more than a lump sum of fish. I will certainly give you more credit for 5, 3" red devils than I will for 15 of them.
5. Recognize that if your fish constantly breeds, so does everyone else's who has that same fish. They are looking to offload their babies just like you. It is for this reason that most stores do not offer credit on convicts, and jewels. The demand is not on par with the supply. However, the same does not go for high quality guppies, mollies and other livebearers which are traditionally heavily inbred by the farms and are of poor health upon arrival to lfs.
6. Recognize that mark ups are different on fish than on dry goods, and that the cost to the stores changes weekly. If I can order from my supplier for .20 a fish, I am not looking to give you .75 a fish. However, next week, my supplier may charge .60 a fish and with freight, it is cheaper for me to get them from you at .75 a fish.
I hope this helps you to understand a little better regarding credit and how/when/why it is issued. Remember, you (the customer) are our best source of advertising. We are not looking to screw you over but we need to make a profit and pay the bills just like all businesses.
-Andrew