I've sold a site and business before. Here is basically how I feel:
1) That, ultimately, will depend on what you are willing to sell it for, and what the other party is willing to pay.
2 and 3) After they have signed an NDA (non-disclosure agreement). Because so many can easily promise not to tell, I also feel--now, 2 years later--that they could put down a deposit of $50-$100, just to weed out the ones who are not serious but were competition wanting to know how much money we made...but, that may have discouraged some from pursuing it who were serious. So, either way...just get them to sign an NDA.
4) You are going to have to trust them, and they will have to trust you, no matter what. A small deposit with an agreement to sell is in order. But, you will need a lawyer also, and they will want to put the remaining amount in an escrow with him or her. Once you have "delivered", you can get the rest of your money.
5) Address this specifically in your contract. This is something you will have to decide upon, as well as what they want.
I can't think of anything else...