You've been selling gold or other item ingame for real money.
I got one a couple of days ago, but I had not been selling gold (yet), so I emailed them and asked the to lift the ban on my account.
Then I waited 4-5 hours, and the ban was lifted
yeah then they thought "hey this guys sells xxxx stacks of herbs...must be a bot user" and bam ban. I would suggest only posting <500 auctions per day.
got a suspension on one account today too for this reason. after appealing it got un-suspensioned. it was a account that i only quest-botted and did some auctions... cloth and this crap...