There can be significant costs associated with the transfer of Trust Points, as well as significant usage limitations if the purchase is not made through an "approved broker." There hasn't yet been a successful Points resale chronicled on TUG, so we haven't been able to figure out if Marriott is conforming to the most extreme limitations as they're written into the governing docs.
And you're right, Marriott Weeks purchased after 6/20/10 are not eligible to be enrolled in the DC. Some of the Weeks available through Marriott's Resales Operations office may be eligible for DC-enrollment, but the prices are of course higher than true external resales.