OP, listen to what everyone is telling you about holding the phone until the check is cleared. If he sends you an email asking you why you haven"t sent it yet (which) I doubt he will. Just tell him your phone is worth a lot of money, and you want to determine if this $800 e check is legitimate and you don't want to get scammed. If he is serious then he should be fine with it, just relax about it though.
Secondly DON'T buy an iPhone 3G from AT&T or any other iPhone. When the next Gen comes out GET THAT.
Yes, it is going to suck not being able to do everything on your phone, but it's only for a month, and you will save a lot of money.
Think about that, if you buy an iPhone 3G out of contract from AT&T (I think is $400) you lost half of your money, and when you try and sell it, you will not get that much, because now the 3GS and the new iPhone's demand will be high. By a cheap AT&T phone from craigslist, and just use that untill the new one comes out. I have done that, waiting for a new phone, its worth it, after using a razr for a month and then getting a shiny new 3GS makes you enjoy it more haha.
Lastly, the only way to really know if it is unlocked or not is to put in a t-mobile sim card and make sure it works, with all the updated baseband and bootloader stuff, you got to make sure it is unlocked. Because if it isn't and you sell him a phone that is not what you advertised, you look like the bad guy. So just make sure.
And $800?? Nice!