4 main causes of that ban.
A) causing a monopoly of Any AH bulk item... and crashing said market. Gets your account flagged... do this often enough and ya it’s ban hammer time, or used to around TBC time at any rate. I dealt with several cases like that blizz really don’t like you flipping items on AH.
B) trading money in game or via the mail with someone who’s been caught selling/buying gold. Even buying/selling numerous items from/to said same person on the AH can result in a flag.
C) being in instances for more than 18 hours in a 24hr period repeatedly. If you accounts log 3 days of 18+ hours a day in an instance they presume you’re botting and out come those flags.
D) You’re straight up buying items for real life money/bought gold.