I have always assumed that they are either the seller or the seller's friend and are bumping the price up in the hope that a genuine buyer will pay more than they would have.I don't think there is any way of blocking them so either the real buyer loses as they pay more (but the seller also gets more) or the seller because the item has not sold.Most annoying.
That's called shill bidding which is also a problem.
I won something on eBay last week and the seller obviously wasn't happy with what it went for as they contacted me and said the item was sold else where.
Elizabeth, I block everyone from my auctions and buy it now items.
You need to block people with negative or zero feedback
Block those with no credit card on file
Block those with no PayPal account
Block those who have had any non payment strikes
Block those in areas you won't deliver too
It will filter all the idiots out.