[mbmb]

"Am I no longer allowed to underline certain passages in any books I own?"

Of course you're allowed to. What you can't do is copy the book, make some alterations, and then sell it.

[NinoFan]

Can I buy a book, make some alterations (say black out lines with a marker), and then sell it at a garage sale? If not, why not? If I can, then please explain to me the reason behind the distinction with respect to the right of the owner of that specific copy(not the author) between the single sale and the large-scale sale.

[FreedomCalls]

Of course you're allowed to. What you can't do is copy the book, make some alterations, and then sell it.

Can I pay someone to make the alterations for me in a book I've already purchased?