I actually thought of the idea of selling my blocks at a profit but for a different reason.
Virgin unspent blocks are more anonymous than blocks that were acquired any other way. If I were to go buy BTC from a brick and mortar Bitcoin dealer that "knows your customer" and then I go use it to buy drugs or something, there's a link. Maybe one wouldn't chase it for drugs, but for something more heinous, maybe.
A freshly generated block has absolutely no history...
How is this different from transferring the coins from an unspent block to a brand new address?
Assuming the anonymity provided by a mined block has some value to the buyer, the miner could just transfer 50 BTC from the unspent block to the buyer's address and receive 60 "dirty" BTC from the buyer.
They just have to be careful that the coins don't get mixed.