Instead of trying to figure out what a dealer's profit margin may be, why not just offer what you think is a good price for the car?
Auction prices can be all over the board. For a desirable (?) low mileage car it isn't uncommon at all for the winning bid to be higher than the wholesale books say. The cars are worth the money and with such low miles the buyer knows that not much reconditioning will be necessary.
You really have no idea what the dealer pays for any used car and that really shouldn't matter to a buyer. It's either a good deal in your mind, or it's not!
There are also costs involved in certifing a used car that can be pretty steep!