I don't mean how to buy them - I mean, how does it actually WORK, in the long run?

Who gets that money?

Because we are swapping books one for one, in theory there are the same number of books in the system as there are credits - but that doesn't work out if people are buying credits - there would be a surplus of credits and an unequal number of books.

Is this just okay because we expect the site to last forever and never have a "run" on books?  Or does the money turn into books somehow?

the money that the site gets when you buy a credit is used to purchase a book to back up the credit. So yes for every credit there is 1 book. 

Oh - so who does that and how do they decide what book to buy?

The site owners R&R and the PBS Team would do the buying and but I have no idea how they would decide what to buy. 

Well there a ton of books on here that just sit and sit so I don't think the surplus of credits thing will ever be an issue. Eventually they get used.  I think PBS uses $ they earn selling credits to fund the cost of the website, fund credits they have to return when a deal goes bad and the offender is no longer a member, things like that. 

You can buy from other members. I sell credits sometimes to cover my postage expenses.  I had a lot of WL books when I joined, including several 2 credit audios. I'm not ordering much because I have so many unread books allready and a cross-country move coming up.  I've kept a large stash to cover WL books while I work through the TBR mountain.

I think that R&R have to have money in their account to back up the credits.  Not books.  But I may be wrong.

That is my understanding as well - that credits are "backed up" by funds.

I, too, sell credits on occasion, and always figured "recycling" existing credits was actually better for PBS than having to back up new credits. Someone who knew a LOT more about accounting explained that my belief was wrong, and that it's about the same to PBS whether you buy credits from them or another member.

What DOES help them, to my understanding, is buddying credits back to the founders, as that means fewer credits in the system that must be backed up. There was a movement in the forums at one time to get everyone to buddy 3 credits back to R&R to cover the 3 we got when we joined.

NOTE: Please, someone who knows different about the credit situation, feel free to correct me.

Interesting!  Thanks for all the info...

I think that they are supposed to have a book to back each credit in the system, if they do not then, they must have the money available to cover the shortage. 

Look in the Book Bazaar Jan. There are usually people advertising that they are selling there. Most want pmt through funded PayPal, some will accept check or money order once in a while. If you don't see any offers you can post that you are looking and list how many and how much you want to pay. Most people sell them for arounf $3 each.