Sellers can place a buyer on a banned bidder list which will prohibit the bidder from placing a bid on that sellers auctions. This is unlikely to happen to you for failing to leave feedback.

Since ebay adopted the silly star system, feedback has gotten kind of meaningless anyway. The bidder's identity protected thing with the asterisks has done even more to hide irregularities (fraud, shill bidding, etc). Ebay makes a ton of money and that's the way they like it.