I've sold points on points.com. Well, not really sold, but traded. I registered virtually all of my airline and hotel reward accounts, then just started doing transfers to get enough to swap for a Best Buy card. At the time, (don't know if its still the case), I could go into Best Buy, pick up a Shell gift card and pay for it with my Best Buy card. Then I buy gas with my freebie. I was able to get a couple hundred dollars by draining all of my reward programs.
I don't really have "credit card points". All my cards allow cash back.