Must sell some of my books. As a teenager I bought loads of old books - some of them 18th century - and some of them could be worth something, especially if I imported them to the US. I have a copy of Ovid's metamorphoses from c.1790 (can’t be bothered to check) with the signatures of owners in Boston and Harvard from the early 19th century.
I donated my only 17th century book to my old Oxford college library, though - it needed specialist restoration and frankly a safer place. I think I paid something like £30 for it in the 80s.