Go aftermarket new, don't get a used one. With a used one, you don't know the truck's history--if it had a blown head gasket, bad sensors or was running poorly for a period of time, there is a good chance the cat has lost some or all of it's effectiveness.
Walker, magnaflow, there are plenty of good brands out there that will be as good as OEM at a fraction of the cost, and certainly better than OEM with 106k miles. It's just exhaust--it's not mission critical hardware and it's certainly not proprietary or complicated. I get that OEM is best for most parts, but this is one area where it simply doesn't make a difference. A good aftermarket brand will be just as good.
Selling a used cat is illegal in CO, so check your local laws. I personally wouldn't touch it with a 10 foot pole