Sell them locally for the best value. Just like a car you can get more selling it outright yourself. Consider this; LFS gives you store credit at let's say 50%. LFS sells your frag for $20 and you get your 50% or $10. Then when you buy from the LFS at retail prices, let's say another frag (with their 50% mark up) you pay another $10 (their cost in this example) plus use your $10 store credit. Essentially, in this example you are giving them your coral plus $10 in trade for one of their frags. Some would say, "Good deal" and others not so much. When you take store credit you should ask for an at cost sale value (or what amounts to a better discount) and this is why; the LFS is making money on your coral once and then making money on your store credit a second time when you buy using store credit at retail prices. If they can not give you a better discount then you are better off selling corals outright or ask for more of the cut of the coral sale price. Even a consignment shop for other types of goods would only asks for 20%. Run an ad in your local yard sale forum to get the word out and they will come knocking on your door happy to get the same coral at a discount for only $15. Then you can take that $15 add only $5 to it an get that same new $20 frag or what ever you wanted without getting taken advantage of by the LFS. Seems like a petty thing but this example is scalable and over time would add up.
Just some thoughts.