Smart move by the rental company. They get free bikes. They probably get to depreciate them since they're rented and then they sell them to you for still more than they would have paid wholesale. I'm not so sure that's in your best interest. Rental bikes would get used a lot. A lot of regular used bikes not so much. Some are. Those that are are probably as well maintained as a rental bike.
I'd suggesting buying an actual used used bike instead of a rental. you'll pay less and the chances are that it's probably been ridden a lot less.