I would have paid the store. That's all I said. It's obvious that other people live by other standards.
It's not a question of standards; The contract was ended.
Even if the sale had been arranged prior between the OP and buyer, and they decided to meet at the LGS to conduct the transaction, the LGS is owed nothing. Perhaps in poor taste to conduct business that way, but certainly not immoral/unethical.
I check cars out for free, and sometimes people go home and fix it themselves. Sucks for me, but that's just how it is, and they owe me nothing. I may charge for inspection the next time they come in, but that would be at my discretion and disclosed prior to the next transaction.
As mentioned, unless the OP signed a binding contract stipulating that he would owe the store a comission even if he sold it on his own, he is under no legal or ethical obligation to give the FFL a single penny.
IMO, from the sound of it, the FFL is the one of questionable character. Most definitely he is greedy/entitled.
As to trading off the OP's rifle; Fine, so long as he would have called the OP and paid him what would have been owed if it were sold. Otherwise, not just wrong, but illegal. It wasn't his to trade. That's theft.