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You can. The going rate I think is around $3/credit. I wouldn't charge anymore than that because then people will just buy them from PBS and help support the site. You can sell them for less if you want but I've always been able to get $3. The easiest way is if you have a paypal account although some people will accept money orders. I personally wouldn't send a money order to someone to buy credits since there's not much recourse if it gets lost or you never get the credits.
When I sell credits, I just post a thread in the book bazaar saying how much I want for them and how many I want to sell. Very often there's posts already there from people who are looking to buy credits and I just respond to that. To transer the money through paypal you just need to PM them w/your paypal email address.