I don't do that anymore, but I can give you some hints.  First check to see how many cars he is allowed to sell in a year before the state makes him get a dealership license.  They keep track of all titles, and add them up.

Try to stick with one line of cars so that parts can be stripped from one for another. Good parts can be stripped and stored, the chassis scrapped to save yard space for another car.  It cuts overhead for manuals and parts if the cars/trucks are from the same manufacturer.  Tag all the parts with good information so you know what they came off of.  You may make more money from parts than you do selling cars.

Use a car dolly instead of tow truck, unless you already have the tow truck.  You won't do a lot of towing, so go with the cheap way.

Finding cars at the lowest price is interesting and takes a little nerve if you use my buddy Tom's method.  Keep you eyes out for broken down cars or cars with tickets on them.  Look in the car's glove box and you will always find some kind of information to locate the owner.  Track them down, and ask what they plan on doing with the car.  Generally you're in a race to get it before the vehicle gets impounded.  Then you have to buy it at the impound auction at a higher price, when you can sometimes get them for free or close to it.  A dead car is a liability to the owner and many people are happy to get rid of them.  The same for driveway weights.  When you see a car in a driveway with a flat tire backed up to the fence so you can't see the license plate, it is a possible purchase.  Knock on the door and ask.

Impounds lots are okay for getting cars if you have the money.  If you want a good trans or motor, find a wreck.  If you want a good body, the motor or trans will probably be shot.  These cars are abandoned for a reason, but they all come with a title of some sort. Don't buy anything with a salvage title unless you plan to strip and scrap it.  You can't reasonably put it back on the road.