What he said ^^^^ in addition, many do not know that you can lease any make of vehicle at any dealership or leasing company. That means you can go to a ford dealer and rent a BMW etc.
Yes, many of these resellers simply purchase the vehicles at auction. Most dealerships do not have the space to carry all off lease vehicles and many only have a certain number of models on hand. This means that say this month, 4 533i's come off lease, they already have 4 on their used lot so they put the least desireable 4 into auction.
All dealers do this but I'd still prefer to buy a used vehicle from a reputable dealer. I've known people who have bought decent vehicles from these small resellers and also known people who have bought lemons from them. I'd rather be guaranteed a good used vehicle than risk a lemon frankly.