Good question.

The distributor that services that team gets first shot at purchasing them. They may take them all or just some of them. I then offer them to the rest of the dealers in this area and then if there is still anything left, it goes to whoever wants them.
I'm responsible for just over 1100 welders between all my sponsored teams. They get swapped out every 2 years. I do half of my teams each year so about 500+ used machines hit the market from me alone. They must be sold because that is part of the money I have to use to purchase the new machines going back in.
So chances are good that I get some machines to sell to the general public but you can't be in a hurry 'cause sometimes the dealers take all they can get their hands on.