No real answer to your question but a couple of points to make.
Pricing for used enlargers can be all over the map and will vary by location. You won't find a lot of C7700MX enlargers for sale in order to do a price check. With respect to the specific C7700 you're looking at, does it come with all the negative carriers and lensboards you will need? LPL/Saunders negative carriers can be hard to find (although I think someone is 3d printing them). Is it well cared for and from a working darkroom or a "garage find"?
As a VERY rough guess, if it is well cared for and comes with 2 lensboards and 2-3 negative carriers as well as the power supply, I would say $300-400 in the US. But that assumes a patient seller in a major metropolitan area.
If its complete (for your needs) and in excellent condition, the worst that can happen is you pay to much. If not, you might pay too much and - on top of that - be unhappy with the enlarger itself.