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...
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If you have an RTH height set to 50-metres for example, that is 50-metres above takeoff point. If you were then flying 50-metres below takeoff point and hit RTH the drone will climb 100-metres before starting its return home.
Do you know about the Feelworld monitors for flying? Ipads heat up and then go dark plus they are not very bright to begin with. I use a Feelworld monitor and it is 2,400 NITS of brightness and does no go dark when it gets hot.