The problem with matching a jacket and pants that were not originally together, is that they may not match in shade. A fabric based on lighting conditions, use, and dry cleaning will fade. It is not just that, even suits were the fabric is from different bolts the shade will differ. Even if you do find a matching pair of pants on ebay that is the same fabric and style, there is a good chance that the shade of blue will be lighter on one of the two items. It will not be a perfect match. I will agree that going back to see if the pants turn up, is a good idea though.
The Brioni was only $7 so it may be worth it to avoid the lifelong trauma I will experience if someone buys the jacket and the trousers turn up later.