Travel credits are dollars, not RR awards. Those dollars belong to the customer, not to Southwest. I don't know why ebay would want to cancel such auctions, since they don't violate any rules. Probably the ebay filter that is designed to detect RR auctions is alerting on one of the words you are using.

Buyers of ticketless funds are at risk of fraud, in that the funds can be recaptured by the seller. Also, if a fraudulent credit card was used to obtain the ticketless funds, Southwest will zero out the funds.