You can check the transaction history of the seller and see if he has gotten a lot of negative feedback from his previous transactions. If he has, it is safe to stay away from him. If he mostly has positive feedback from a number of buyers, you can be sure of his credibility.

Similarly, the Square Travel Certification you mentioned is also an indicator of his ownership.

The most important thing you should check out is if the seller has paid all the necessary fees on his timeshare or not. A lot of times, people buy timeshares through eBay and then find out that the seller hasn't paid the maintenance fee properly. So, you need to verify that as well.

At the end of the day, you need to be really, really careful about buying/selling timeshares through eBay. You need to spend a lot of time to make sure nothing goes wrong. That, in my opinion, is not worth it. So, I'd suggest buying from a good timeshare company. The choice of course is yours.