Part of the issue, ESPECIALLY when its a trade (and not a "I'll pay you this $$ for this gun" deal) is the expectations of either party.
Some look at "talking" as just that-- talk. While others will "hold" the item until the "talks" are complete and a yes/no decision has been made.
The best practice here is to communicate VERY carefully with the seller. Something like "I'm interested in your item, can you put it on hold for 24hrs to see if we can reach an agreement?"
Then, if an agreement is made, each party must stick to it until the transaction is complete. Failure to do this would then be grounds of negative feedback.
However, if no agreement is struck, and no "hold" has been negotiated, then the seller is free to trade it off to whomever they do make an agreement with.
I can't blame him, unless I specifically agree to hold an item for you. I would have done the exact same thing. Maybe he traded for the same gun you were offering to trade, but with someone else who could make it happen sooner.
To be honest, if you didn't have a prearranged agreement, then it's his prerogative to move his item at whatever speed suits him best. That speed may or may not fit your preferred speed.
I know how it feels to have a perceived deal fall through the cracks-- it sucks. Just learn from it and move on.
In the future, make all communications crystal clear. Say "I really would like to make this trade work, but I'd like you to guarantee me first rights while we negotiate." Then the seller can decide if he wants to be confined to those parameters or not. But at least you two would be aware of each other's intentions up front.
I have no problem with rush trades and have made quite a few. For me personally, I have a crazy schedule and if the days I have to utilize don't work out, depending on what happens in my store, it may be weeks before I'm available to meet for a trade again. You never know the guys reasoning why he wanted to trade quick and he might be in a similar boat as me...?