If you purchased a liquid asset with the $10,000 (and your creditors never found out about it in time to sue you for it), couldn't you just reliquidate it at that point and use it to pay off the IRS debt, leaving you with pure profit (and a horrible credit score)?
Actually, wait, maybe not even that: given that you purchased the debt, you could legally mark it (as the creditor) as having been repaid and then petition (as the debtor) to have your credit score corrected, no? This is what the http://rollingjubilee.org people are doing, at least.
Repaying negative status debts does not necessarily improve one's credit score. You can ask the credit reporting agencies to reflect the fact that you've paid it, but this is a no-op for virtually everything except mortgage applications.
Actually, wait, maybe not even that: given that you purchased the debt, you could legally mark it (as the creditor) as having been repaid and then petition (as the debtor) to have your credit score corrected, no? This is what the http://rollingjubilee.org people are doing, at least.