Commodity should not be on the vocabulary page. (I was assuming the suggestion to put it on was a joke.)
It's not a Bitcoin specific term, and isn't relevant to Bitcoin at all, other than so people can argue about what it is/isn't/does/doesn't apply to.
Yeah, he was joking, but I think it's a good idea and can be done non-controversially. People have to go to a bid/ask currency exchange to buy or sell bitcoins over the Internet. General market trading terms are very related.