The largest auction for these vehicles is by far at Manheim Auto Auction in Manheim, PA. Manheim is the largest auto auction in the world. (Although I believe there is a very large one out west somewhere too... in Vegas maybe??) Manheim owns auction houses all over the US, but their flagship is in Manheim, PA.
In PA, every other Thursday, they hold their 'Exotic' car auction (aka 'highline') with the vast majority of off lease high end vehicles being listed here. They sell/run an average of about 8k cars per auction at this sale. Those that do not sell, are higher miles or stragglers (ie are too late to register for thursday), run on the Friday open auctions. There are auctions everyday, some for only specific brands (ie GM or Ford). Most BMW's, Audi's, Saab's, mb's etc are run at the exotic sale.
We used a dealer/broker. Our dealer does not have a lot (car lot that is
) and purchases vehicles for specific customers who have made specific requests. They will search for and purchase a car at auction to your specifications. They are onsite to inspect vehicles presale and handle the postsale transactions, temp tags and shipping (if necessary). They charge a flat rate up to a $$ max, then a percentage of the transaction price. This is well below what any dealer would hope to make because of the lack of overhead in their business. They are really a 'service' industry as opposed to a true 'dealership'.
For us this worked out well, although, to be honest, we put sweat equity into the purchase. We researched extensively about these cars ahead of time (thanks in large part to the awesome and thorough information on this board) and while our dealer was well versed in general about the car we were looking for, we had more specific information on the exact model we were interested in. We took an active role in the purchase of our specific car (talking on the phone through many, many presale inspections, deciding on bid max's for specific vehicles, taking into consideration age, miles, condition etc). Certainly far more labor than looking at a car lot taking it for a spin and making a decision. On the other hand, we feel we saved significantly on the cost of the vehicle we obtained because we were patient, selective and stuck to our criteria.
Hope this information helps!