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...
As someone who works closely with DJI folks here in the U.S., I can promise you they have not left the US market "unprotected and in disarray". They are working very hard to get CBP to cease stopping shipments. DJI has answered every question and...
The only time you're not allowed to fly from National Forest Lands is when they are "Designated Wilderness Areas". There are also times when TFRs are in place, or an area may be close due to nesting seasons.
Other than that, it's legal. Just...