Used books come with a history. Sometimes it's a history you'd rather not advertise.
Worst is the underlining and highlighting usually found in textbooks or assigned reading college books... There is not much you can do with these except disclose the defect: "Some highlighting throughout." "Some highlighting, not much." Etc.
If there is writing in the margins or underlining...I sometimes say: "Some insightful Marginalia included at no extra charge!"
Ex Libris...? "Delightful Ex Libris on inside cover" Sometimes I describe the Book Plate especially if its vintage, or fits the book in some way. I sold a History of the Coast Guard with a Rockwell Kentish "fish" motiff Book Plate... This is a plus for many buyers.
Owners signiture on the inside cover or FFEP (First Free EndPaper)...? I'd Say: "Prior Owners signiture FFEP."
Or, "Prior owner's notes inside cover."
Some people don't sign their books, but have stickers or even embossed seals!
Others sign their books in unusual places... I had a couple sign the title page...not the author! Another signed the first page of text...! Go Figure...
If a price...usually low like 50 cents...is on the FFEP...and you've sold the book for $20+...sometimes s discrete black marker just the size of the offensive price will do the trick...
This also works for those odd: "Not for Resale" stamps you see in some books... Why raise unneeded questions in the minds of the buyer... Why is this book "Not for Resale"...? Who Cares...it was probably "comped" to someone at a university or at a book fair...it's still the same book.
Assuming it's not an Advance Reading Copy (ARC) or Uncorrected Proof which are often considered to be "hyper-first editions," even though Amazon won't allow you to sell them... Some collectors will pay extra for ARCs or Uncorrected Proofs...especially for block buster best sellers.
If a book has really odd marks on the FFEP or inside cover sometimes one of your own "Ex Librises will cover the defect...although occasionally the defect will still show through on the other side of the FFEP. But a new ExLibris does dress up a used book at times.
On really old books check for dead insects...moldy backs and water damage...always check the back of the book inside the DJ...that's where they hide...
Bumped corners can be hit with an appropriate marker to "color" them back up... Really scuffed corners can be "re-built" with a dab of bookbinders glue that drys clear.
Then alas, the "remainder mark" which is usually on the pages edges...but often hidden near the spine...nothing you can do.
Also beware of "remainder spray" which is a marbling look on the page bottoms... No it's not decoration! Although I think it sometimes looks nicer than a black mark!
With old library books that are stamped up and rebound...sometimes there is an extra FFEP which if removed...will rid you have 80% of the Librarians "damage" to the book. A vast improvment...and only at the cost of a sharp razor blade...!
All this extra effort can make an old book look better than you found it...
