There are several issues you raise here. First, you can always limit your rentals only to renters who are able to transfer points to your account (ones who cannot is only because those points have been previously rented or banked, but doesn't mean they are not trustworthy). And you can always protect yourself from scams by renting from "verified" renters, and asking the renter to show you a screenshot of their account with the points and their expiration date listed. I've been on both sides of transactions multiple times on vacationpointexchange and things have always been super smooth and quick. That doesn't say there isn't fraud on there - the site lists multiple instances of that, but I suspect it's a small fraction of all the transactions that happened there and common sense can protect you.
This is really a good point. If renting is not that appealing to you then you can buy 3000 destination points right now on Redweek for $6000. It will cost you an extra $9000 in fees to Marriott to get them to be part of the system. But that's still less than $20K (and you get to keep your gold week which you can use as a week or points).
Frankly, IMO the only reason (for an informed buyer) to buy points from Marriott is when the buyer also has post-2010 resale weeks that can also be enrolled in the points system as part of the transaction. Even then, if one already has access to the points system via other enrolled weeks the value is questionable, but at least you are getting in the transaction something you cannot otherwise get by buying resale.