If you are sensing your trader opponent, and you are in a group of pirates, one of you can always play shield for the cargo and protect it from being shot. I did that a few times and it worked.
I, personally, dislike the people saying 'I won't give you your cargo, you can't carry it anyway' (because I would be smart enough to sell the pirated cargo to an allied smuggler or a junker); same for traders just destroying their cargo solely so the pirate can't have it, in my eyes, it's grieving and also demotivating for a pirate, especially those, who RP their role.
Also, go ahead and shoot the trader, when he shoots your loot.