Swappa does not sell phones. Swappa is simply a marketplace, not a seller. What you meant if you were trying to analogize is that Walmart is not a swappa seller. Thanks for pointing out the obvious.
Walmart is selling used phones just like swappa sellers are. Walmart however is lying and calling them refurbs when they aren't.
Walmart could pay to have the phones properly refurbished but instead, they clearly mark them as locked to AT&T:
https://www.walmart.com/ip/iPhone-6-16GB-Refurbished-AT-T-Locked/42104299
Just like a swappa seller would.
Walmart also sells unlocked used phones:
https://www.walmart.com/ip/Refurbished-Apple-iPhone-6-64GB-GSM-Smartphone-Unlocked/49230864
OP bought a locked used phone from Walmart, no different from buying from swappa except that swappa sellers don't lie and call their used phones refurbs when they aren't. OP could have bought an unlocked one, but chose not to.
I have already posted those links before. You need to read the thread before you jump into the conversation so that people don't have to repeat themselves.