if you're working and an avid gamer. i suggest you get an ipod and another phone. because if you use your iphone for gaming, the battery would not last more than 4 hours (if you play continuously). less if you use internet too.

if your battery is dead, then how you're going to make calls and smses?
First, check if it is the real iphone. dont get tricked into with a similar looking china-made "iphone"
Ask for warranty,
Check the 3G, Internet, Wifi,speaker, and the home button
If the seller have service card from telco, match the IMEI (serial number) on your iphone and on the card, make sure its the same.

If possible, ask the seller to use his SIMcard on the phone to check everything first. you will need a microsim for iphone instead of a normal simcard.
and when you want to deal, make sure, you pay and you get the item he showed you.

if you suspected there is something amiss, dont buy the phone.

Better reject it and don't buy it.
It's too good to be true.
800 for iPhone 4 is way too cheap.