Using these?

Wait, do I use it before purchasing the goods?

Yes.

Well now I feel stupid

I thought you only had to use it before selling your goods.

That works as well, the bonus of the "buff" is applied when selling at least one item. The Ducats you get on use will be received regardless of when you use it. It's like a Shadow Crystal, you can use it either before the mission or while you're already in it.