Well, I'm not defending your LFS, but it's possible they were not so wrong
in not taking back your fish. Many shops do not want to take back your
problem fish and transfer it to either their own tanks or another customers.
My LFS wouldn't take back some Africans from me for the same reason. He's
had plenty of times when he took them in, sold them to another customer,
only to have them attack or kill the customer's original fish. I couldn't
blame him. He was only being responsible.

And again, at a different LFS last week, they didn't want a really big
clown trigger I have. It would easily sell for two to three hundred. I
told them it was nipping away at some of my other fish and was considering
getting rid of it. Being large and agressive, they didn't want it since it
would need its own tank or only ending up attacking their fish or another
customers. I couldn't blame them either. They are very responsible with
the fish they sell and question and advise every customer before selling a
fish. So, in both my cases, the LFS were only acting in the best interests
of their shop and their customers. Perhaps it's for reasons like this
that they no longer accept trades.

Other than saying they changed their policy, did they provide you with any
sort of explanation as to why? I'd imagine that aside from the above
reasons, that perhaps shops might be worried about bringing in a diseased
fish from a customer as well. Who knows, I'm sure they have their reasons.
It's a shame either way. For me, half the fun is going to all the LFS
nearby and finding the fish I want and sometimes at great deals. Mail order
just doesn't seem fun to me and generally after shipping, not any cheaper.