You need a dealer's license to attend these "Dealer Only" auctions. This is usually not worth your while just to buy one car. You must pay to establish a business and get a tax code. You must also pay for a dealer's license. This cost would exceed any amount you would save buy buying a car from auction. Dealers make out buy buying several vehicles and passing off this cost when they sell the vehicle.
Also, many Dealer Only auctions are starting to limit how many people they can bring with them. They are getting smart to too many people buying from these auctions that shouldn't be there.
Also, these will not have any warranty. The only exception is if there is some remaining factory warranty.