Q:

How can I check that used or old books are in good condition before I purchase them?

I go to used book sales about once or twice a year. Recently, I bought some books that I've been meaning to read for a while. However, when I started reading them, I realized that about a quarter of the books I purchased weren't in great shape -- no missing pages thankfully -- but pages that were about to fall out, a broken spine, brittle paper, etc. I'm not really sure how long they'll hold up for.
I obviously should have checked the books before I purchased them. To be honest, they were really cheap, so I was more focused on getting as many books as I could for fifteen dollars (my usual budget) than I was on actually making sure that the books were in good condition.
But for future reference, what should I look out for when purchasing used/old books?

A:

I know the expression is "You can't judge a book by its cover," but in this case, you might be able to start there.  :)
If you have time to look at each individual book for a minute or two, I would personally start by checking the cover for evident "trauma" marks.  I mean like big dents or crushed corners (typical if the book is dropped).  These are frequently what will wear on the binding of the book, which is typically what matters to avid users.
The second thing I would check is to open the book at random.  Is there a place where the book tends to fall open very easily?  If so, check the pages next to there.  Are they starting tear or disconnect along the seam?  This could be a sign either that someone left the book on its face with a weight on it, or simply left it open for a long time - both of which will weaken the binding, making pages more prone to fall out.
A third thing I would check is page wear.  This is a somewhat unusual issue, but on the occasion, especially with very old books, you'll run across a copy that was very used - to the point that the pages are starting to wear away on the corners.  While this can be nice for antique collectors, for a reader, it's not ideal, since the pages are more fragile and will tear easily.
There are other things too, and I'm looking forward to seeing what other community members come up with, but that's what I tend to check for.  Hope it helps!

A:

In addition to @anonymous2's excellent answer, look for water damage. Check for stains, on the pages, the outer binding, and the dust jacket if there is one. Look at the edges of the pages for mold spots. Ruffle through the pages and sniff. No really. You can smell mold or mustiness, which will tell you if the book was wet or damp and developed a mold problem.
Also, if the book is not considerably old or of historic value, put it in a freezer bag and stick it in a freezer below 0ºF (-20ºC) for a week. This will kill any bedbugs (and hopefully any other critters which may have nested in it). 

