I'm actually interested in knowing if I bought the debt of someone else, could I write whatever I wanted on their credit report? For example, could I purchase $15k worth of debt for $500 and then forgive the debt and report to the credit agency that it was paid in full?

You can't erase the statement that the debt was 180+ days late and sent to collections.

The original debt holder could erase that statement, but they have absolutely no incentive to, since they took a 97% loss on your debt.

A single late payment like that will probably drop your FICO score 150-200 points.

Not true.

I walked away from my primary residence because it lost over $100K in value. My credit score only dropped from ~705 to 655 (per Creditkarma.com). Also, only one of my creditors lowered my credit limit (Citibank). American Express didn't even blink, and I've got a Platinum card with them with a $75K+ limit. To date, I've missed over 22 payments on my mortgage, and my credit score still remains fairly usable (I may complete a short sale, or may let it forclose. Depends on how amiable the noteholder is to both scenarios).

You've been lied to. The risks of walking away from most debt is highly overrated.

In US, most debt, you never have to pay off regardless of whether its forgiven or not. It's not a crime, and its not hard to avoid collectors.