I remember being highly impressed by a sign over the door (or a plaque on the wall, or something) which said something to the effect that 'There has been a bookshop on these premises since XXXX', where XXXX was a long time ago. 16th century? 17th century? I don't dare to guess now but Wikipedia tells me that the university was founded at the beginning of the 13th century, so 16th might not be too far-fetched for a bookshop there.