Originally Posted by Jozawun
What you are saying is that it is ethical to buy a 2nd-hand out of print book and to make a copy for your own purposes without further compensating the author; but that is unethical to borrow the book from a library and make a copy for your own purposes unless you do further compensate the author.
Yes. Spot on.
If you buy a 2nd-hand out of print book and retain it, you reduce the supply of 2nd-hand copies of that book. If the second-hand market runs low and demand increases, some publisher might investigate the possibility of a new print run.
Again, look at what happens if everyone does it.
If people buy up second-hand copies and retain them (even if they also make personal-use copies as well), the chances of a new print run or a new edition are quite high.
If people just borrow the existing copies from libraries and make copies for themselves before returning the book to the library, there is no chance of a new print run or a new edition.