Ask HN: What's stopping me from buying my own debt?

Let's say I owe a hospital a $10,000 debt, and they've sold it to a debt collector for $9,000. Now, that debt collector is looking for a buyer for that debt, as they tend to make the rounds if they don't think they don't think they're able to collect from the debtor. Sometimes they'll go through several debt collectors.

What's stopping the debtor in this scenario from purchasing their debt at the "debt market" value?