No. There is no proof that Gox is selling BTC it doesn't have, yet. People assume that they are insolvent, but as I said before, only karpeles and his cronies know for sure.
The truth is no one knows, but the market is acting on pure assumption at this point due to a lack of communication from Gox.
What does gox achieve by not allowing withdrawals?
Conspiracy to drive down the price?
Although, Karpeles looks like a complete moron, so my bet is, he has lost the money, screwed everyone over and is now destroying gox.
Because they haven't fixed the transaction duplication problem.