Mel, if you sell those snakes and pocket all profits, where Alex sees nothing (gets nothing) from it, that is the action of a thief. If you give nothing to Alex, even if you keep all snakes, that is also the action of a thief.
This is, all, still a civil matter but you are teetering on the edge of it becoming something criminal by law (it's already ethically criminal).
It appears you breached the second agreement (second agreement amends, or voids, a first agreement/contract). You have provided no proof otherwise. For that, or even regardless of that, you should either send Alex the gun or pay for shipping back all snakes you received from him.
To Alex,
If you do not receive any compensation, and prefer the snakes back, I believe Mel should pay for shipping back to you, or pay half at the least end, but it seems that will not happen.
If I were you, I would just pay for shipping and be done with it. At least you would have the snakes whose value is much more than the cost of shipping. Plus, the cost of shipping is much less aggravating than going the route of a lawsuit or other.
I, also, would not not allow him to keep "the big girl". All snakes should be sent back to you. Mel's reasoning, of how he should be able to keep her because he had to buy a cage for her, is laughable. The cage purchase was his decision. Plus, if he wants to use that excuse, you are also out money due to having to pay shipping both ways. He, at least, would have a cage, to re-use, while you would be back to "square one" though having lost some money (what it cost you to ship).
Next time, you should check the laws/regulations on whether, or not, you are able to possess what you want to trade for.
That or, perhaps (if firearms), get proof that the arms were delivered to the FFL (on the other person's end), and all paperwork is complete, before shipping snakes.
An added insurance, IF FFL dealer is willing, is he/she could have instructions not to ship the arms until/unless he/she receives proof that you shipped the snakes.
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