You don't really need an escrow if the transaction will be in a face-to-face manner. You can just bring a friend and meet up in a public place where there are lots of people.
Anyhow, There's a right board for buying/selling bitcoins, you should move this thread there:
Currency exchange But I'll be recommending
https://localbitcoins.com for this as you'll have more chances there.
Thank you very much。But i think its More secure ，deal with law firm or bank， cos we need 50000-10000 BTC.
If the buyer is really big guy and you are unable to believe him still using escrow will not gonna make you die. That is actually good for you since the middle man is handle both the party transactions mate.
Kindly ignore the direct dealing dude, if possible you can use the localbitcoins.com website to get the bitcoins for the fiat cash on banking deposit or instant transfer both will be work better.
I would prefer to transact through escrow ,, use trusted escrow if the transaction you involve reaches 50000-10000 btc ,, because it would be very risky if you use escrow that have low feedback ,, i want you to give this opportunity to zazarb escrow for transaction you will do