Of course an attractively framed picture of a famous and interesting painting will always be sellable.
But lets think about how age might add some value over and above what you might call the intrinsic decorative value.
I think it's important to think of the age of the print relative to the age of the painting. I think it fair to say that as a rule of thumb the closer together the two are, then the more likely the print is to be valuable.
So for instance, consider the 16th Century painting known as the Mona Lisa ... yes, the one everybody has heard of, this has been produced as a print for almost 500 years! Which means of course that there are a considerable number of them around, none of which would be particularily interesting from a collector's point of view unless they were old copper plate engravings from within a couple of hundred years or so of 1520 (which was when Leonardo da Vinci completed it)
Any thoughts?