This is a bit random, but do publishers like Penguin move a lot of classics each year? Middlemarch’s rank in the Amazon USA list would seem to indicate it sells around ten copies a day. Does that seem reasonable? I thought it seemed a bit low, but my opinion is truly random. There are >12 different paperback editions, so that would seem to argue for popularity…
Speaking as someone who’s worked in both new and used books, classics are purchased far more often from used retailers than new. I worked at a massive used bookstore and we couldn’t keep most of the classics on the shelves, but at the new retailer I worked at I found they only really sold if they had a fancy binding or cover.