Perhaps ask for Celestron's help first before writing off the SLT mount. Or as you say, it could the chance for an upgrade.
You'll have no problem selling the OTA on its own. The scope has a standard Vixen dovetail, so will attach to whole host of different mounts. Perhaps you may want to do that yourself instead of selling the scope. Its a great scope, identical to the Skywatcher Skymax 127. The C6 is also a great scope, we have one, it replaced our Skymax 127 many moons ago. I only did it at the time as the C6 was cheap and the 127 sold for the same money. Otherwise i wouldn't have bot