At a Marriott sales presentation last week, the Sales Person made the statement that Marriott Point System units are not offered for resale by anyone other than Marriott.
As with most statements like this, its really hard to reply without knowing the context of the conversation. If indeed, by "units" you are referring to points, then what the others have said is true, you
can purchase them resale on the external market. If by "units" you were discussing a deeded week and wanting to purchase one of those today and have it be part of the points system, then there is likely some validity to what the sales peep was saying. The only way to currently purchase a deeded week and have it become part of the points system is through Marriott and only by also purchasing a like allotment of Trust points which corresponds to the quantity of points allocated to the deeded week. Currently, you cannot purchase a deeded week on the resale market and enroll that week into the points program. That ship has already sailed.
Figured it was worth tossing that point out there as not being privy to conversation, I was not 100% sure exactly what context his/her comment arose and your use of the word "units" instead of points made me think perhaps you were talking weeks, not points.