Example (not saying this is fair, just an example)
Buyer pays 10 BTC down, purchase price of 40 BTC.
All mining profits go to original owner towards 40 BTC. When 40 BTC is paid unit is the buyers after paying for shipping cost. If the mining does not pay the remaining 30 BTC, buyer needs to make up difference + ship to get the unit. Buyer needs to pay for running electricity costs.
Yes, this is the one ways how might work. Anyway the electricity is on the buyers side, in generally this means someone is mining for you. It's just like buying shares, but with start decision to sell unit.