Unfortunately you are at the mercy of whoever is going to take them from you. You are talking something not very rare so I'm leaning towards getting 30 to 40% even if that from a LFS trade. Also size doesn't really matter and many want smaller fish to begin with.
Before you take in the fish explore the area to see what the LFS is selling them for. Get an idea and this becomes your starting point or high water mark. Now look online from one or two sources. This is your low price marker.
Now you have the information and can shop around your fish for trade in. It isn't going to be just about price. It is going to be also about what goods they have so you can use your money to buy something else at a lower rate. Fish food, corals, hardware, etc. No need selling the fish to a store you don't want anything at. That makes no sense.
Explain what you are doing, their age, show the ware, let them offer a price. Now you know what they are selling for and can safely counter. They will have to decide yes or not. If they say no you can offer another price. Or let them know you are willing to trade for food, corals, or a different fish.
Nothing is set in stone in this hobby.