[Dobby]

Well maybe no so much nice as explanatory. When he said the he didn't accept anything but gifts as a form of payment, you could have said "Well I'm not from the United States and as such I cannot send the payment to you as a gift, but I do have solid enough reputation to where you can trust me to send it in another form of payment." If he continued to be an ass about it, by all means start to tear into him, but the misunderstanding of your inability to send a payment as a gift was what led to the conflict and overall failed trade, and if you had made that clear initially he may have gone along with the trade.

edit: spelling