Actually, if you killed him and he bought a new ship, came back and attacked you (after only 40 minutes), that is against the rules, as it is re-engaging.
basically its good not to ask a trader for more then his cargo size in thousands, (say asking a transport for 1m, or an adv train for 5m), there is no actual rule about this, its just courtesy.
The only thing is you must ask for a reasonable amount. No asking lvl 35 traders for 5m etc, that can amount to terrorism.