There was no official ban on possesiion per se (as I explained in my previous posts) but a lot of this stuff was printed in France by an émigré dissident publishing house, which the government propaganda crtiticized from time to time. I guess that created an impression of a "forbidden fruit", even though much of their pubs were available in academic libraries, at least better ones. Another issue was a very low number of copies (usually between 1 and 10 thousand) of foreign pubs that were translated and sold by domestic publishers. That created shortages, especially of popular titles, which further increased the demand.
Wojtek