You claimed that on your income tax return, of course 
He’s right though, it’s all about buying them right. My feeling and experience is that it isn’t worth the hassle for $500 cars. Especially when it costs $50 in gas to go pickup something with the trailer even if it’s close by. They are risky at that price level. You win some and lose some. If you do enough you’ll come out ahead. It just takes some time.
Another tip, try to sell mainly on swap sheet or Craigslist or something. You have too many cars in the yard with a for sale sign over the course of a year the town code officer might own you.