It's worth noting that more often than not, used cable modems are getting to be more of a crap shoot. More and more cable internet companies are starting to register the modem MAC address to either an internal or central shared database, and they aren't de-registering those device MAC addresses after closing accounts. This is to combat theft and aftermarket sale of rented equipment, but it also catches and hurts legitimate second-hand equipment activation as well... and there's no recourse to fix the issue but to return it and buy new.
Buying a new cable modem is awful from an electronic waste standpoint, but that's not entirely on your head. Buy new, Motorola/Arris is fine, but aim for the cheapest DOCSIS3 model you can get away with. When you leave them, be sure to de-register your modem with them to ensure it can actually and safely be resold (in theory).
This said, I don't think Charter Spectrum allows you to save any money by opting out of using their equipment, and they're currently looking to tack on an extra $5/month base fee for everyone this month.
However, I'm not one of those people inclined to trust ISP controlled network equipment for my router/WiFi setup as I'm sure you've noticed from the other thread, and tend to neuter those routers and put them in passthrough mode in favor of my own. YMMV.