Please do no not purchase private keys from anyone in lieu of creating a UTXO, regardless of the reason. Not your keys, not your coins.
I believe the proposition to buy the coinbase reward, which as far as I know would have to entail buying the private keys. Once you send the bitcoin, it's no longer coinbase. It would be cool to own coinbase coins, one of the reasons to buy Ballet's titanium blocks, as you get bitcoin with 0 history.
All that being said, I can't see anybody taking the risk with the whole coinbase reward, 50 bitcoin. You have to trust the owner will never just use the private key himself.