Wow this thread started today and already 6 pages. Sorry had to flip to the last page but i probably got the whole story.
A deal is a deal. Its like selling a card on ebay, the buyer pays for it. And then someone else message you saying that they'll pay you more for it. So you just refund the payment and sell to the other guy. Very terrible thing to do, and no point bring it up that its "going to a collector instead of flipper" Thats just an excuse for yourself.
What if the "collector" can only offer you $60 and the op still offered you $75, i bet u would sell to the op without a blink.