As others have said, yes you have to register it.
Can you sell your photos? It depends.
If you're going to go out and fly to get photos for sale, then yes you need your Remote Pilot Certificate (a.k.a. 107). If someone reaches out to you after...
Walgreen, Walmart, and other big-box photo services provide a decent product for the price, but output is regulated by an algorithm designed to produce average results over a wide range of anticipated exposure values. It's a one-size-fits-all...
I most
If you can afford it, get an Epson or Canon that prints tabloid size. It's nice being able to experiment with different papers, and have the ability to print tests without waiting.
For outsourcing prints, I'd look at at these guys...