M.L.
Facebook actually has a store setting for pages, so no, it's not illegal to sell on there, when they are the one's providing the tools to set up a shop.
The only thing that people can't do on facebook is do giveaways, which businesses still do all the time. If they have a blog or something where the giveaway is being hosted, they can announce it on fb, but they can't actually hold the contest there.