Love the auctions, bought lots of equipment from them. But there is a difference. With an auction purchase you have to be prepared for repairs and save enough money to make the gamble worth it. You normally will not test drive a boat at auction. Many times they won't even start from having dead batteries and bad fuel. Who knows how long they have been sitting with ponds of water soaking in the hull.
Private party purchase you may be able to do a test drive and hear about the history and/or prior problems that a boat may of had.
I really think there are bargains to be found with both types of purchases. But buying at auction can be worse than buying from a used car salesman. Good luck and know the max you want to bid before the item comes up.