I don’t mark my books because I give away or sell them (in Australia, I can normally sell most books for 1/3-1/2 of what I paid, which goes straight back to more books!)

It does look like fun though, especially on TikTok you see these complex annotation systems that people use.

Since you are in the UK, you might check out Bookswap. They look like an affordable way to get physical books. The alternative is FB marketplace

Both, but mostly buy books (of purchased books, I’d say roughly 50/50 new vs used). My library seems to have a very limited selection of newer titles so I’ll buy newer physical books and borrow available ebooks from my library. Saves a trip to the library and I have a nice waitlist of books shaping up