What causes the doubt? It looks like the initial plan was for the seller to collect the money then send it the charity, which I would say is possibly dodgy. But, after the bids were getting high, he said:
"For whoever wins, I think it makes sense for you to donate directly to Celiac disease and send me the receipt - this cuts out the ebay/paypal middleman, and let's you collect the tax deduction properly."
So there doesn't seem to be any doubt that the buyers money is going to charity. This doesn't rule out the possibility that someone really wants the bag, but I have to think they have the charity in mind.