My Kenmore is rock solid when standing on its three little legs, but I attribute this mainly to the fact that is cast iron and it weighs around 40 pounds!
I agree that you should offer the case. Explain that it's not a very good one, and leave the decision to the buyer.
You never know, they may be a collector, or be trying to find a machine just like the one their mom taught them to sew on or something.
That type of buyer may appreciate the case even if it's for sentimental rather than practical reasons.