Not all TUG ads are placed by TUG members. There is no requirement for membership to place an ad.
To protect yourself, ask for verification that the renter has the right to rent that week. A copy of the most recent maintenance fee billing. Or a copy of the reservation confirmation (from the resort). Or a copy of an ownership document. Etc. Simply ask for proof of ownership, suggesting some possibilities. If the renter refuses, move on to another ad.
Also insist on having your name added to the reservation or inserted in place of the owner. Call the resort to verify that. Whether the renter will agree to do that before you make final payment is for you and the renter to work out. You don't want to be stiffed, but neither does the renter!
Here's a major reason, other than getting scammed, for asking for proof of ownership: Beware of someone who offers to rent a week obtained from one of the exchange companies - as an exchange, as a Getaway, as a bonus week, etc. Many exchange companies, including II and RCI, prohibit renting weeks obtained through the exchange company. The danger to you is that you might show up and have no place to stay because the exchange company has cancelled the reservation.
Finally, if you still have concerns and if the renter agrees, you can engage an escrow company to hold funds until you have proof that you'll be able to use the week. However, you'll likely have to foot the bill for that service.
You should also consider using a rental agreement. The renter might already have one ready to use. Alternatively, you can use one of the agreements in the Renting section of TUG Advice (link at the top of the page). Many such agreements include provisions for providing you evidence of ownership, the resort's confirmation that you'll be able to use the week or some other aid in helping you to get comfortable with the arrangements. Thus, using such an agreement will save you the trouble of separately asking for all of the suggested info.