I am from Brazil, I dunno how it works in the US, but here, "junk cars" are for illegally bringing back to streets illegal cars for some reason or another.

Here the system is very simple, yet efficient and lucrative: You steal a car (for example a BMW, here in Brazil those get 2, even 3 times the US price).

Then, you buy a junked BMW, maybe even a totalled one, with roughly the same model.

Then you "mashup" the two, mixing the two cars into one, making sure the serial numbers on the chassis and engine end being the least illegal ones.

Then you bribe some officials... And you can resell your "refurbished" and "used" BMW for the US market price (that here is 1/3 of the normal price), you can even claim to clueless buyers that you bought the car in the US and is reselling here (this is illegal here, reason why the car is so expensive, you need a special permit to buy a car outside Brazil), to make them think the car is legal.

Of course, since you made the car using two cheaply obtained cars, this is very lucrative.

Some people don't even steal cars, if the model is popular enough (And still lucrative) you can just keep buying junked cars until you have enough parts to build a sell-able one.