The bus shelters usually get covered in posters for textbooks for sale. I wouldn't recommend putting them up anywhere other than bulletin boards since they'll get taken down much faster each night by the cleaning staff and just waste paper. If you want to put up posters, get one stamped by the Underground and then photocopy it and post a few up in the building where your books will be most attractive (ie. ENG books in engineering buildings).
When I was selling my books, I found online to be the best way to sell them and get responses. Posters really are ignored by students these days for the most part, those bulletin boards get so cluttered most don't read them. If someone is looking for a specific book where are they going to go first? Wandering down the hall looking for posters? No. They are going to search the internet and McMaster used book marketplaces such as
MacInsiders, Facebook Marketplace, and
Titles Used Book classifieds (not heavily advertised since Titles prefers you use their Buyback service so they can make more money rather than a free classifieds where they make nothing).