They are definitely compensated via a gross profit commission split just like others at the dealership. They do have volume bonuses to make up for the fact that most of their deals will be low or no profit, but if you inquire on the internet about buying a rare model, it's not like they're going to discount it drastically and get a low commission... It's a rare model after all.

YMMV, but I got it directly from a roommate who worked in internet sales at a Ford dealership. Volume >> profit.