said by pflog:
When you change your MAC address, you will get a new IP address. That can change routing and more importantly can then allow you to bypass filtering or firewall rules for the destination sites in question.
Yup, all info that I knew before I posted. As mentioned, queries to both sites were answered, and neither has implemented any IP filtering or other rules that would be blocking me. My interactions with these sites is limited to benign posts and purchases.
I was hoping that others have had the same type of thing happen and might have a better understanding. Either these sites are lying to me and are blocking my specific IP for some totally unknown reason, or my guess is that it could be a range of IPs being blocked by a provider downstream.