It is an auction site, but you may want to look into it as opposed to the store credit approach. The store buying your books is may still undercut so they can make a profit. With a site like Half.com you're directly selling your stuff. You can always check and see if some books get more money than others on Half, and only sell the ones you think you'll make the most on.
Then again, I've never looked into Barristers or anything like that, so maybe they're very fair/generous?