Selling vehicles is hard money. It's a pile of stress and big ups/downs, and the only reason to do it is to make decent cash. Other than that...it's a tough job with a ton of BS, tons of hours, 0 benefits and people who don't understand why you won't work for free. I've never met any sales guy who's moved from any other type of job to selling vehicles and think it was easier, or shorter hours, or less stress, or better time off, or anything remotely along those lines.
Now, I am too cheap to trade things a lot of the time...so I clean it up, take pics, build an ad, and do it all myself. But if you don't want to put in the time/effort, it's going to cost you, just like anything else. Just the clean up job I would do to sell my personal vehicle would be $300-$400 alone at a detail shop. The dealer needs to account for cleaning, servicing, repairs, pay a guy to build the ads, pay the sales dude, and still make some profit. Big numbers don't mean anything for profit...you could have a $10k unit with good profit in it, and a $40k unit you're selling at cost. Depends on a lot of things.
Plus take on all the risk that the thing might sell in a day or not for 6 months. Huge risk in owning a car dealership...definetly too much risk for not making good profits. Anyone who would buy a car dealership to make an average wage would have to be on acid.