CIL Imperial shells were what I and many older folk would tell you were the the finest ammunition made period. The Special Long Range shotgun shells were the only shotgun shells I ever bought and used when I first started hunting back in the mid 70's. These had higher (longer) brass which allowed them to have more powder and make them into a magnum load back when only 2 3/4 ' shells were the standard as 3 " shells were not available.
Some of my first hunting trips were with my uncle who lived in Sudbury and the CIL Imperials reigned supreme. I know for a fact that back in those days people who guided for ducks and especially geese would gladly pay a premium to get their hands on these as they were not as readily available in some remote areas and certainly not available for sale in the U.S. to my knowledge. Any American who travelled up north to hunt would also pay the premium for these.
As you know, these shells were loaded with lead shot which may make them somewhat restricted for use, however that being said it would not surprise me if they would still be snapped up none the less.
The plastic hulls were also top grade and great for many reloads. I traded a few remaining boxes of mine that I had been hanging on to for some years for some other center fire ammo from a dealer at a gun show. The dealer knew the reputation of the shotgun shells and would have no problem selling them to his contacts up north as he told me. I made out very well in my trade with him.
I still have some plastic boxes of CIL .22 LR ammo that I purchased around the same time and they shoot very, very well. Much better than most of the bulk stuff that is out there these days. It is a shame that CIL closed their munitions plant years ago.
My advise, don't be too anxious to sell them and settle for a quick buck. There are collectors out there to consider and you never know, they may increase even more in value in a few more years- supply and demand as they say.
fortuna