My local only takes marines by donation which is a bit of a PITA, but they do as you say with fresh water. Best thing is to sell or swap online as you get to meet a whole bunch of like minded people and will get alot more than your LFS will possibly give you.
Also its not a bad move to give some of the easier to keep things away to people starting out in the hobbie, really seems to steam people on and keeps them motivated after such heavy outlays. It amazing when some has just spent $5k on a set up and get a free $10 coral frag how happy they are.