Is there any requirement on how many books you have to have posted in order to keep your account & credits? It's just that almost all of my books have been requested right away, and I can't afford any more shipping right now. :/

NOpe - not after the inital 9 are posted - some keep none at all to trade - they just buy credits when they want to order

Ok because I'm broke for the next couple of months! I've sent 17 books in just a couple of weeks! *whew*

you can put the books you have left on vacaton hold. 

Wow! Great! Just think of all the new books you can request with all your credits! You can put your account on hold til' you catch up.

 There's no limit or minimum number.  I've had from 10 to 70 and back down again on my shelf!  Also, you can put your account on "hold" if you have a postage budget.  I do that from time to time.  I budget $20 monthly for postage. When it's gone, I put my account on hold until the next month.  Works great for me!