I don't take notice of my IP address much, so I can't say if they assigned a static IP address in my case. If they're filtering on IP addresses, I imagine that they get a lot of false positives.
If they are filtering on IP and you're not paying for a static IP address, then Cox's DHCP server could have given you a new IP address that matched someone else who had been banned. Not out of the ballpark of possibilities when IP is used for selection / banning.
Would you have accessed it outside of your home at any point? It could be possible that Amazon keeps a record of where you log in, you shared an IP with a previously banned account at some point, and then you listed an item the other seller had listed.
I'm sure there's a simpler explanation, though. That seems fairly complex.