Fist I pretty much only buy second hand books. When I pick one up it is usually because:
A. I already know the author. (Though it doesn't mean I have read or liked anything by them. I have a list of 'I must get round to reading...' in my head that just gets bigger and bigger.)
B. it has a semi-naked, large-breasted woman on the cover.
I then apply the Mcluhan Test and turn to Page 69. If Page 69 is shite I put the book back. If Page 69 is readable I may flip to another page at random and read that. (I have a suspicion that authors know about Mcluhan Test and deliberately write really good page 69s.) If Page 69 is really really shite and looks like we're in the realm of ''so bad it's good". I may flip to another page at random. The words 'Hilarious' or 'Trilogy' anywhere on the front cover and the book gets put back.
I then look at the price.
Often if books are cheap enough, boot sales, 50p boxes outside book shops, or market stalls, I will buy books just from the spines alone.
Sometimes by the armful.
If this happens, books are then carefully (but quickly) stacked with the crappiest, cheapest ones on top and presented to the vendor. Occasionally, very occasionally but it does happen, the vendor will look at the pile of books in my arms, look at the price of the first three books then just count the rest and say something along the lines of, "Oh... just give us a fiver for the lot." At which point I will agree, thrust the money in his hands, and leave before he notices the book at the bottom of the pile was priced at £6.
I try not to punch the air while I am still in the shop.