Upload & Sell: Off
For the photography I do (landscape and architecture, no sports, no weddings) I'd much rather have a 1DsIII in excellent condition for say $2,400 - 2,500 than a 5DIII for say $2,900 unless one of the features of the 5DIII (e.g. faster fps, higher iso, brighter viewing screen, etc.) was important to me (which it isn't). Focusing is at least as good and as fast, image quality is at least as good, and you're getting a pro camera built like a brick (of course it weighs almost as much as a brick too : - )). Full disclosure: I own a 1DsIII and have no thought of selling it and buying a 5DIII.
Obviously nobody can really answer your question. But my guess is that the used prices for the 1DsIII will remain about where they are now until a successor to the 5DIII or a similar new camera in the future is announced or until the prices of 5DIIIs drop significantly.