OP I totally understand what you're saying (I'm not trying to troll), and I think that a store that specializes in cichlids is fantastic, I'm just saying you might want to have other things in addition. It's true, many hobbyists come into stores and are enamored with the cichlids, but 90% of the time they are incompatible with the fish they already have. That being said, many other fish species will work with many cichlid species, and people like to incorporate variety into their tanks. For example, if you're selling African cichlids, many people would love to be able to buy Synodontis cats to add to their tanks. Kribensis cichlids usually work well in a community tank, and discus look fantastic with schooling fish. Not to mention, it's easy and advisable to sell dither fish for certain cichlid communities. And just about every freshwater customer out there wants a pleco or other algae eater. People with snail problems will want loaches, people with hair algae may want flagfish, siamese algae eaters, or amano shrimp.
And if they have to get rid of fish to get cichlids, they are going to want to trade the fish in. By not accepting trade-ins you are only hurting your business. Most stores will often offer store credit for trade-ins at about 1/2 of their sale price (depending on markup). That way the people spend money in your store, and they spend it on products you have marked-up. Take this example:
Assuming both of the products (fish traded in and the new fish) are valued at $10 retail. If everything was a 2x markup for example, they would trade in their fish, getting $5 credit. Then, they spend their $5 credit and $5 cash to buy a new fish, which you paid $5 for. You have just broken even, and you still have a fish in your tank valued at $10! (spent $5 wholesale price for your original fish, gave $5 credit, and collected $10 from the sale.) From a trade-in you have actually made more than you would from a regular sale, and the customer is happy because they saved money and ensured that the fish will have a new home.
Furthermore, if another store gets the trade-in, the customer will be forced to spend money at that store. If you're lucky they get a cichlid there and come to you for the rest. If you're unlucky, they've stolen your customer.
Just my 2 cents from my experience. Best of luck and feel free to ask any questions. That store I was talking about (Marine Warehouse) has an incredible cichlid selection, but they've been in business for over 30 years because they have everything else as well.