1) selling items need take time and waiting response
2) mob stop to attack and attacking mob without damage during hunting mob (some time char already die after get response from server)

i already tested my internet speed and no issue found.

Yup, yesterday for example during leveling few members of my party (including me) were facing connection issues with the server (as you said everything around us just stopped responding). The issue were not on our side since we play from different Europe/Asia countries.