I try to only mark if I think it'll be useful to me, or to someone in the future. I'll lightly mark the location of favorite passages in the margins (that's useful to me, at least, and minimally intrusive), record some information about some obscure reference to a location by an antiquated name that I was only able to track down in some decades-old humanities paper, mark up the TOC with information I wish it had, that kind of thing.

I don't mind most marks from previous readers. Usually I'd rather have them than not, as they're at least interesting in one way or another.

The category of used book with annotations that I don't ever like to buy is one where a previous owner highlighted or underlined seemingly half the book. There's a kind of reader out there who must highlight or underline their books the same way I compulsively select text as I read on a screen, and it wrecks the book.