If anyone come to me asking for a gift trade (only happens when there is some icon reward to be had for it), I tell them to gift me first.
If they cannot come good on it for whatever reason, or "have to wait a couple days till i get my paycheck", too bad for them.
I even have had a case of one guy on EUNE acting all friendly, needing help with his PC. So, sure, I helped, and after a handful of hours we got it sorted. He were promising me a skin numerous times throughout the discussion, though I never asked for something in return (we agreed on Elementalist Lux in the end).
Come the release date, he became all "sorry, we can't afford it right now. maybe later". I just replied matter-of-factly "~17.50 EUR", to which he responded "you can have the skin if you don't want my mom to buy food for a month".
At that point, I didn't care, and just removed him. I can't stand liars.