I have a multistep process.

If the book is really old where the ebook is free, I just read the ebook.

If the book is newer than that, I check the library first. If not available at the library and not available as a cheap ebook, then I buy the book.

Used books vary a lot in price. If the used book is only a couple dollar savings over the new book, I just buy the new book.

I'm 40 and own a few thousand books. Those are just the ones the library didn't have that weren't available free or cheap.

Now, I do make some exceptions for absolute favorites. So, books I'm likely to reread or pick up occasionally I go ahead and buy a copy.

But, yeah, if you read a lot you can quickly own way too many books if you buy them all.