It's my understanding that the MA AG has made the rule that internet dealers must be licensed to sell items to MA. They just don't want to go through whatever pain in the ass process licensing requires. Same reason why next to no one will ship ammunition to MA. even though it's perfectly legal to buy ammo online.
On ammo you are correct, that is the AG's position . . . and that there is no mechanism for a non-MA business to get the permit she believes is required.
As for anything else, you are confused.