Since there's some interest in points, here's why I might offer a Cat 4 certificate vs. offering to sell points:
Seller perspective: Marriott has been rolling out Mega Bonuses for a while and I'll earn one of those certificates after 2 or 3 one-night stays. On the other hand, although I travel for business and am Marriott silver (almost worthless), I tend to stay at low priced hotels (avg. under $100 room rate) and often at Residence Inn which earns at a lower rate. So let's say I earn, on average, 1,000 points per night. That means 30 nights to get me the 30K Cat 6 reward. Now if I sell a Cat 4 certificate for $90 I'm getting the equivalent of a $30-45 return per night. If I sell 30K points for something like $250 then I'm getting the equivalent of an $8-9 return per night.
Buyer perspective: There are plenty of Cat 4 hotels that go in the high $100s plus tax. Buy a certificate and save $100, or 50%. But Cat 6 rarely go over $300 plus tax, so spend $250 on the points and save only $50, or something like 20%.
I know, I know, there are plenty of exceptions and opportunities that could change this, but that was my general cheshbon.