If you mean Paperback Book Swap its really simple. You post books that you want to trade for other books. The cost is the mailing of books. We use is at the library primarily to fill holes in series.
For example, I post books that I want to trade. It could be a copy of "The DaVinci Code" that someone has donated. Someone requests the book and I send it to that pbs member. In return I get a credit.
If the library's book #3 in a series was checked out and never returned, I request a copy from another pbs member. I spend my credit to get the book.
This is very helpful when for some reason book #3 is no longer in print. Usually, the cost of mailing my book is less than buying a paperback version of the book the library needs.
I hope that helps.