The Voucher Terms and Conditions state,
"4. Southwest LUV Vouchers and travel issued in exchange for Southwest LUV Vouchers have no cash value and cannot be exchanged for cash or other monetary consideration. Southwest LUV Vouchers will not be replaced if lost, stolen, or destroyed.
6. Southwest LUV Vouchers are VOID if sold, altered, purchased, brokered, and/or bartered."
So, you're not supposed to sell them. Having said that, you could give them to a friend or family member, and if they wanted to thank you with a nice Christmas gift, that would be up to them.
I recently gave the remainder of my voucher, which would go unused otherwise, to a friend, who had no issues using the remaining funds (I used about $240, so he had about $60 to work with). And, no, I was not compensated for my generosity, I just wanted to pay it forward!