For the $25k scam you mentioned to work, a large portion of the loan would have to be for sale at pennies on the dollar (I don't see that happening).
How many pennies? If you're looking at the prospect of getting 10-20 cents on the dollar in collections (if that) and you could sell for 60 cents on the dollar, I think a lot of lenders would do that.
If 50% of the lenders sell their $12500 stake in the loan for 60 cents on the dollar ($7500), the borrower pockets $5000. Very possible and very tempting.
And who's to say that it's a "scam" anyway? Good luck proving that.
The borrower will say he hit a rough patch financially and couldn't pay for a while, then got some good luck and was able to pay off the loan.
The guy who buys at 60 cents on the dollar, when confronted, admits to being a friend of the borrower, but says he only bought the loan because he knows his friend is an honest, hard-working guy that would catch up and denys that they are in cahoots.