Where has SEGA warned people not to buy items from certain users? Because I never seen such a thing besides "don't RMT/cheat" announcements. Obviously you shouldn't buy from suspicious shops with 10+ stack of each AC item-- that really should be common sense, since you're just funding their RMT operation, but I don't think a perm ban is warranted. Temp ban/wipe of said item? Sure, why not.
I've bought massively undercut AC items from the shop, but only from "legitimate looking" users. (like 1 or just a few of said item in the listing) I avoid those sellers like PN2743848966 (WHO STILL IS NOT BANNED) and usually just throw them on my blacklist.
Will I get banned for my purchases? I sure hope not because it's really hard to tell if an undercut is from a "bot" or not if it's just a single item. Sooner or later these "bots" will smarten up and stop listing their entire AC scratch stock to look more "legitimate"-- They may even start setting their player name and setup their shop ad with text and a random Symbol Art. If SEGA doesn't fix this shit, we're gonna be seeing even more bans then. The players should not be punished for their mess that they are too lazy to fix.