Transfers are a bit different than rentals, since once you transfer the points, they are gone. Still, if it's to a legitimate
DVC member with a decent number of postings on disboards, it's hard to imagine it being a scam. The deadbeat payer seems more likely. Still, the safest way might be to use a company like escrow.com, which will take the money from the buyer, but not transfer it to the seller until the deal has been completed. I think it's usually used for goods auctions, but I think they would probably do timeshare rentals or points transfers too.
Redweek.com provides escrow services for timeshare rentals, underwritten by first american title. You may want to look into this. It's pricey though -- 10 percent or so.
Caveat: I don't know the terms used by either of these companies. They might be renter friendly or owner friendly. You'd want to do your own due diligence.
If it were me, I'd probably just get a bunch of info from the person buying my points -- home address, phone number, etc. I'd probably do terms like: You pay half up front, I'll transfer all the points, and then you pay the rest. That way, both of you are taking a 50 percent risk. Probably depends on the amount of money involved too.