If the resale has already been worked out, I think it's treading dangerously close to a field of straw.
Let's say that you've mentioned to me that you'd love to have gun X, but can't seem to find it anywhere for a good price. You say that you'd gladly pay $Y for that gun and tell me that if I ever find one at or below that price to go ahead and buy it and you'll buy it off me. I happen to come across it for less than or equal to $Y and buy it, fully intending to sell it to you. The only difference between that and a true straw purchase is that you're giving me the money after I buy it, not before. The deal was already worked out beforehand. That, to me, is masking the actual owner and would be a straw purchase. Sure, no money has changed hands yet, so it's not the very definition of a straw purchase, but it sure is close, and not a subtlety I'd want to hang a defense upon.