Unfortunately this kind of thing happens all the time in auction houses all over the world, but its usually the auctioneer who instigates it, in order to get the bidding up and over the reserve price (otherwise the auction house gets no sales commission). The trade calls it 'taking a bid from the chandelier', as the auctioneer literally points to the back of the room and takes a bid from thin air.
I am pretty certain that some sellers do this on eBay, but I don't think its illegal. I am fairly certain that the rules are (at least in the UK) that no-one is obligated to purchase except the high bidder, and if this falls through, the seller may contact other bidders to do a deal with them. I purchased this way on a couple of occasions, and have never paid any more than my high bid (NEVER the original winning bid).
Basically, the upshot is that (if it IS a true bidder) if this seat post is worth that much to them then good luck!! My father (an antique dealer) always taught me that you should write down what you are prepared to pay (what it is truly worth to you to have it), and never bid above it. You may miss out on a few 'bargains' here and there, but you won't get caught up in a bidding frenzy either.
Postscript: I have also recently had a buyer for my Raleigh Chopper on eBay who was fictitious - my thoughts are that it was a seller of another Chopper, who bid high for mine under a false e-mail address, so that my potential purchasers would bid on his/her auction instead. Infuriating, I know especially as it made me look like ICHIY to the under-bidders, and of course I can't prove anything!!
Chris