The Internet is filled with socially inept people because the one thing that keeps these people somewhat in check in reality isn't present on the Internet (contact). Just write off this person's complaints as those of someone craving the attention and approval that they don't need but are told to by society that they do; while possessing none of the skills that are really the pre-requisite for them to do so.
Better yet, don't just ignore him/her; report him/her to eBay for harassment, but whatever you do don't actually respond to them.
That being said, I have some inkling of where he/she is coming from. I just got through an auction where I was outbid by 50¢ twice and then at the same time the auction ends, I get an eBay message from the seller telling me that the high bidder backed out and that I was being offered the lot for the price I had last bid. I knew then and there what was going on, but being the collector and consumer whore (thanks again, society!) that I am I bought it. Seller: 1 - Steel Rabbit: 0.
Normally I figure out what the most I would ever pay for an item would be then I bid that amount and eBay bids up to that for me, and if I don't get it, then it obviously was out of my price range, but this time I didn't follow my model and I got grifted. But really, it's not the end of the world, I got the item for a reasonable price, and the shipping turnaround was quick. I would've just liked to not be treated like a rube.
So yeah, contact eBay, and ignore this person's disparaging comments.