If you're selling them to the WUSA bookshop you can mark or write in them. They're you're books and you can do what you want....but the more writing and highlighting the less you're gonna get for them.
If you sell them privately, as in putting a notice up on a notice board with your number so people can call you, you can do whatever you like to them, again it will just dictate how much the buyer is willing to pay. You'd obviously pay more for a textbook that although second hand, is in very good condition with minimal-to-no markings, than a second hand one that has been 'well-loved' so to speak.
As far as selling them back to the unishop goes this is the first year they've introduced it and I highly doubt they'd take back textbooks with writing and highlighting all through them.
I guess it depends what degree you're doing too. You can't really give away you law textbooks because you need to refer back to them in later subjects and they will be useful to some extent in your career. Same would go for Commerce subjects I would imagine.
But if they're just random books you're not going to need again, it probably helps to recoup the hideous expenses textbooks can bear if you sell them on.
