probably over 80% of the Adv. Trains I come across think they are tough enough to kill my Greyhound. They all fail. If you got rid of your missile load out, you'd be able to kill them too. Once a trader opens fire on you, the rp is over and the engagement begins. It's true traders should not be exempt from the roleplaying before an engagement rule.
Even though Halt is not a demand, you don't have to wait for a trader to stop before asking them to pay for passage.