OK so buying credits from PBS cost $3.79 pre credit and includes the swap fee of $.049. Net cost $3.39. Why would anyone buy from other members for $3.00 + $0.49 swap fee? Just can not figure how the mathe makes any sense.

If you are an standard member you don't pay swap fees. Or perhaps, better to say that unlimited free swap fees are included in your membership.

Probably because they want to help other members have $ to send out books.  Many, if not most, sell credits for that reason.

3.79 including the 0.49 fee is still more than 3.00 plus 0.49 because that is only 3.49. If you are buying a credit for 3.79 and it includes the 0.49 fee, the credit cost you 3.30 and the fee cost 0.49.

Then there is the 50 cent transaction fee for the PBS credit purchase, so the $3 that other members sell credits for seems like a lot better deal than $3.79 from PBS.

If I wanted 3 credits (you need to buy 3 to get $3.79 price) that is $11.37 then the transaction fee makes it $11.87 so that ends up being $3.97 a credit. So almost a $1 more to buy directly from PBS.

The 50 cent kiosk fee is a flat fee, no matter how much is in your cart. If you buy, say $20 or $40 of PBS Money, you'd pay one 50 cent fee. You can then use the PBS Money to buy credits when you need them without incurring another 50 cent fee, so most members don't pay a 50 cent fee every time they purchase a credit. (They buy it with PBS Money.)

Some members have accumulated Free Swaps (which replace the 49 cent Swap Fee) by mailing books with PBS Postage. Some are Standard Members, as Sara said, and I think Anna's explanation is true, too. Lots of people here like to help each other out. But the $3 credit is a good deal, as Sarah explained, even though it doesn't include the Swap Fee. There's a slight risk, I suppose, that someone wouldn't deliver on the credits, but I haven't heard of that happening.