A little off-topic, but I used to work at a major auto parts chain when I was going to school about 10 years ago. One of the local shops would buy a lot of reman'd parts from us: starters, alternators, etc. He'd charge the customer the full price for the part, including the core. Plus his markup was 25% as I recall.
Once a month, he'd come in with a truck bed full of cores and pocket the cash.
I'm sure it's prevalent, but I was shocked at the time.