The above image is pretty close to what my local Paper Back exchange looks like, but with about 10,000 more books in a smaller space. Mine is called, "The Book Rack." I think it's a chain because there are about three of them within 20 miles or so. Most of the science fiction and fantasy are at the one closest to me, for obvious reasons.
In my last pick up post i mentioned reselling books and this is pretty much what I meant: I will buy books that are .10 a piece that are not romance books (Danielle Steel, Fabio cover books), political or biography, and children's books. These books they will not take because they just have so many. I'd say 75% or more of the customers of these places are old ladies reading romance novels. As I had mentioned earlier, the economy is not what is was, so the owners of these places work the counter. They have changed the rules as well; once upon a time you could get 100% store credit for all trade ins, and now you get 50% which is not that bad. With over 5000 science fiction and fantasy novels to sell, I can always find something I want.
Prices of the books are almost always half of the cover price, unless they are old, then they are $1.50. So 50% off here is needed if you're looking to pick up books you will not be reselling later. The lady that owns The Book Rack near my house is super knowledgeable about rare and valuable books and has almost certainly out of boredom looked all the old books up on eBay, and sold them.
The above picture is what angers me about paper back exchanges. Fucking stamps. They stamp the book where the hell ever, typically inside the front cover, but not all the time. Several times I have seen stamps over the dedication or sometimes in the actual writing of the introduction... it's hit or miss. Sometimes the people who shop here bring the book from another exchange, in which case there will be three or four stamps. For shits sake. I can see your formula, it's half the cover price. I can pretty much figure out what is half.
I don't mind a stamp, honestly. Just put it somewhere that will not affect the value. Sometimes I stamp my books when I loan them out. I have a library kit and program on the computer for just such a thing. I won't do it to a book I'm intending to sell on the other hand.
In conclusion, try not to rip off you're local exchange, but feel free to pick up as many books as you can when out garage selling. Look for the best deals in bulk and bring them in for some credit. Get a good large credit going so when you see that whole series on her shelf you never noticed before you can pick up the whole thing for half price. In comparison, I always have more than $100 on credit at any time. I often give $20 or so away at a time to friends to use if it gets any higher than that.