There is basically nothing you can do if theirs account get banned, everyone knows here that account sharing is illegal and can get you in trouble. ( Not sure what the actions are on d2jsp once this happens though )
Ive done multiple services over the years and no accounts got banned, simply dont use a bot ( this is a very very high chance of getting banned ) and play as human as possible
Use keybinds, dont play with away/dnd status, put some stuff on ah just for the lulz , talk with random people ( even though buyers dont want you to )
Ive never used VPN myself and never got in trouble
Also since youre new use mediators for trades, its safest way for both users