Frankly if we are going to get this market off the ground we have to trust SOMEBODY.  I like your guidelines.  If you know where and who the person is, you have a leg up in litigating your funds back (since bitcoins do have an estimate-able value, theft is a criminal action.)  Another thing you should require is a merchant agreement.  When you are making a purchase online you want there to be a page (or better yet, receive and e-mail) that says exactly what product or service you are receiving and the cost in bitcoins.  You also want this to include the deposit address for the wallet, so you can prove that you paid to that address the specified amount from your transaction history. (print off the webpage.)

Another important factor is if they accept cash transactions.  In the United States and many Western Countries cash is legal tender, required by law to be accepted for transactions.  If they do not have a method for you to buy their product using a cash method, such as paypal, credit card, Western Union, etc, and operate solely on bitcoin then they are operating illegally.  Even if they are legitimately offering products and services you are taking a gamble that they won't be closed down through legal channels before filling your order, and probably shouldn't be trusted anyways for their complete lack of business sense.

Edit http://www.treasury.gov/resource-center/faqs/Currency/Pages/legal-tender.aspx Apparently private businesses in the US are not required to accept cash, but it would definitely be a step towards their credibility in my opinion.  Since fraud with legal tender is a federal offense and raises the stakes quite a bit for criminals.