I saw this mentioned on another thread where it was being discussed what to do with gift cards to stores you don't want. Someone said that you could use the Book Bazaar forum for that, to post a gift card you have and see if someone would swap for an Amazon gift card or PBS credits or a Wish Listed book.

I've only ever seen the Bazaar used for credits; can it be used in that way? I have several gift cards I'd much rather trade for Amazon gift card or PBS credits. 

Gift cards can be used to buy credits, providing you find someone willing to do that -- but the Bazaar is book/credit-related only!
From the Book Bazaar forum :

Topic: How to use this forum - please read before posting here!

You can sell or buy PBS credits and have the payment be with a gift card. But that is it. They cannot be swapped in there. For anything. 

They CANNOT be offered for a wish list book. Attempting to jump the wish list lines is not allowed and you will get your membership suspended for attempting to buy or sell books here.

Whereever you saw that thread, please let people know in it that the Book Bazaar cannot be used for those purposes. They can only be used as a form of payment for buying credits.

In a local Von's supermarket yesterday, I noticed there was a machine to obtain cash for gift cards. If I hadn't read this thread, I'd never have noticed at all. 

I was on my way out and had too many cold/frozen food items to spend the time to find out how it works, what % is charged, etc..  I imagine it's substantial, but might be worth looking into. 

I know their 'coins to cash' machine retains 10.9% - I prefer to give that to one of the younger kids in the family for wrapping the coins.