Only get it when
– you'll read it at least 5 times.
– it's a reference book you'll use a lot.
– you need it under a time constraint (i.e. now) or must have it longer than the loan period at the library.
– you're planning to damage it during use (e.g. flour-filled baking books, workbooks, bathtub reading material).
– you plan to donate it to the library.
– to support the author.
Otherwise, use the library!
Discardia 
You could change the title of your post to “When to buy a book NEW.” For anyone who shops at used book sales or rummage sales, standards can be a lot lower.
For example, I went to my local library’s used book sale yesterday and they were selling paperback mass-market books for 10 cents. I bought 20 for $2.00 — cheaper than the cost of gas to drive to the library once! When I’m done, I just add them to my “donate” pile so someone else can read them. I still go to the library, but it’s nice to have backups for when you don’t have time to make the trip there (my nearest decent library is a half-hour drive).
Also, for voracious readers, the library doesn’t always have every book you want (and many libraries charge a fee for interlibrary loan). I use Bookmooch.com to trade books I have for books I want but can’t get at the library.
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I just picked up 5 books (all Penguins!) from the local St Vinnie’s for $0.25. They will get looked over and then sent through my favorite used bookseller (Books By Chance) who will do their best to sell them on Amazon, and if they don’t sell I’ll donate to the library.
I’m finding that with my local library I can get pretty much any new book ordered if they don’t have it, and quite a bit of literature through interlibrary loan, but every so often there’s a book I really want that has a multi-month hold time for copies and then it’s time to buy.
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You left out one…when you live in a town with a local library that carries very little of what’s important to you.
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