I don't think they can trade atm because gox internal transfers are no longer working ...?
They could trade now, as if they're honest they could decide to only trade existing goxcoins in the bitcoinbuilder gox account. This would only be a number in the bitcoinbuilder database reflecting their balance with mtgox which is not accessible.
Essentially they would be trading coins that might not ever be payable, but it still would be kind of interesting as it would give an indication about the market sentiment as to whether mtgox will reopen or not in some shape or form.
Legally if this could cause any trouble, I don't know. Perhaps if someone bought mtgox btc at bitcoin builder and then mtgox never reopned, bitcoinbuilder could be sued? I don't know.
