Thinking about ideas we could trade with the Druchii Sorceress:
-Spices and luxury goods: they're easy to obtain and would be very expensive in Naggaroth.
-Books about the enemies of men (Skaven, Orcs, Chaos etc.): not much to say, in general less of these is something the Empire wants.
-Queekish: if neither the dwarves nor the Asur could figure out Queekish in thousands of years I doubt the Druchii would have. I think we could get a massive price for it.
-Books about the Empire, Bretonnia, etc. (I mean common information, history, etc. (Leaving out coastal information and all that)): a bit riskier, but at the same time I really don't see how they'd be able to exploit said information in a meaningful way without expending massive resources. Because at the end of the day, we're still separated by an entire ocean very patrolled by the Asur.
-Ithilmar: we could buy a bunch of Ilthimar from the Empire and sell it to them at an extremely premium price. I don't think a single Ilthimar shipment would do much at the scale of a country as big as Naggaroth but I'd pretty much prefer to sell them to the Eonir even if we can only get money from them.
-AV: while it sounds like a bad idea, if we only sell them 1 or 2 gallons and no research I don't think they'd be able to do much with it. As in, for all AV is amazing they'd be starting said research tree from zero.
Something I also want to point out is this part:
Very few of the cuts your Empire is forever bleeding from come from Druchii blades. In fact, I believe that much more of your blood has been shed by the riders of the bastard steeds to the west in any one of the wars you've fought, than has been by all the Druchii Corsairs throughout history, and yet you ride to war alongside them when circumstances dictate you must.
For all the Druchii are horrible people the truth is that the Empire has lost and was hurt more in almost any war, waaagh, etc. than against the Druchii through all of history. So no matter what we trade with them, I doubt it'd end up impacting the empire or nations of the old world negatively in a meaningful way.
Also something to point out is that we'd not really be making an exchange with Naggaroth, but with a single sorceress who is here behind Morathi's back. So considering that and how Druchii society works I think it's most likely the Druchii sorceresses or her mistress who is not Morathi use this to advance their own positions than to actually better Naggaroth as a whole.
Plus, we would get things in exchange too which could benefit the Empire: more books about Enemies of Men which could be put to use when the next Waaagh arrives or the next time the Skaven attack, or about Chaos just in time for the next Everchosen, books about shipbuilding and navigation for the Empire's navy, or about Lustria for the poor mayor of Swamp town or even about Chaos sorcery with the only purpose to learn how to counter it (which again kinda useful with an Everchosen around the corner).