Bought a brand new 18" center console boat/motor/trailer in 07 from an Ebay no reserve auction.
I haven't bought a boat (got a couple off Craigslist though), but I've bought 2 vehicles, a motorcycle, watches, scuba gear, etc. off Ebay. I even buy batteries off Ebay. I've probably bought close to 500 different items off it.
As far as the seller bidding the price up, who cares?! I don't. If the price gets bid up to high regardless of who's doing the bidding, then move on to another item. The seller is still going to get charged commissions for selling that he will have to pay and sometimes listing fees as well. It's in his best interest to get the most that he can, but if he does win his own auction, it's gonna cost him fees to Ebay, so it's a gamble for the buyer.
Ebay is a really unique system for connecting buyers and sellers from around the globe. You just need to understand how the system works to fully take advantage of it.