If your goal is to get more money for a prime, I'd say go with a 6D instead as another poster previously stated. That's actually exactly what I did and do not regret it at all.
For the type of shooting you said you do, there really is no "need" for the 5D3. From an IQ standpoint, I also prefer the files out of the 6D for whatever reason. Ever so slightly, but I do.
Also, depending on the deal you get, I'm certain you could probably pocket around 800-1000. I sold my gripped 5D3 earlier in the year for 2600 and bought a 6D for 1400. No brainer for my situation.
The other issue I see with you swapping out for a 7D2 is that it completely changes your lens collection. If one of the main things you do is landscape, you will surely miss the full frame sensor and not having to deal with the 1.6x crop. Even putting landscape aside, it will change the way you use your lenses for everything.
Is there a specific feature of the 7D2 that you feel is necessary for what you do?
Exactly. Selling the 5DIII and picking up a 6D in the $1250 - $1350 range will leave you about $1200 for a nice prime, such as the 135L (about $800 used) and funds for a nice flash.