So he isnt botting then just playing quite hard. FAOD it doesnt mean hes playing hardcore during those entire hours. More likely he is relisting and undercutting.
It can show him on during that time but he could just be using the ah feature from his phone.
Logged in doesnt always mean playing.
Follow some of the above strategies dude or find another market. On the basis of what you ahve told us, then I dont believe he is doing anything wrong.