your btc don't leave when you place an order, after an order is matched, then you send the btc. I am not sure if orders will stay matched after expiration though.
I think we have the same question...
What happens if the XCP sale offer expires before the BTC arrive? Are you now at the mercy of the seller to return your BTC or choose to honor the trade? If I have understood correctly, Counterparty can only escrow XCP and assets generated inside its system.
I think yes, you are at the mercy of the seller..
That would make the examples I saw of trades (not real ones) with expirations of 3 to 5 blocks very tight, no? Especially when you account for the randomness of block generation.