Quote from Reddit: Its highly unlikely they ban people for selling on the market even if the buyer has bought gold since market sales are more or less anonymous while player to player trading is more personal since you had to have contacted the buyer (or seller) (Or know them beforehand as in a network of gil sellers) in order to do the transaction.
They could have simply flagged you since personally traded with someone that was flagged for buying gil.
Sort of like guilt by association. They froze your account to make sure your not doing anything wrong and stop you from profiting from your account if you are.
It sucks but if you didn't do anything wrong I'm sure everything will be fine.