I recently purchased an account but it has an untrusted negative rep...
It is for the previous owner selling another account. I see alot of people selling accounts, but they aren't -repped.
Was this guy just trolling?
Trading accounts would result in a negative ratings for a long time. This somewhat changed a little, but I think there are still many that consider it shady. The main argument against it is that you can buy an account with a positive trust rating and thus indirectly buy trust. A scammer could buy a neutral rating which is still an improvement over a negative rating and could use the bought account to scam again.
Without having read the actual feedback though its hard to tell whether it was legit or just trolling.