Though the thread has been opened for half a year, I may want to share my experience after buying the reward tickets several times.
It is always a great value to buy a Southwest RR ticket esp for last minute travel. But I guess some steps are necessary to prevent from buying from scammers.
1) Make sure the ticket is really Southwest Rapid rewards ticket. You can check in "change reservation " website on southwest.com. If you see the reward availability instead of normal booking availability, then it's reward ticket. The best way is to meet the seller in person and ask him to show that the reservation confirmation code can access the reward seat inventory on spot before payment.
2) Once you know it's a reward ticket, you will not be scammed by those who use fake/stolen credit cards mentioned in the Google search.
3) The next possible risk is that the seller cancels your ticket after your payment. You will also see this problem even though you meet the seller in person. So what you have to do is to make sure you make the payment via PAYPAL even though you meet the seller in person. This serves as black and white for your transaction and if there is anything happening, you can always file a dispute to PAYPAL and ask for chargeback.
4) Given that you need the protection from Credit card/ Paypal, your claim must be done in 60 days after your purchase. It means that your flight should be departed no later than 60 days after your purchase so that you can utilize the protections. Actually it makes sense, as the ticket prices after 60 days normally won't be much more than $375.
Hence, with all this step, you should be safe to go.