Make sure at the least you know all of the above.
There are no hard and fast rules about buying second hand products, but mostly common sense and getting the greatest idea possible about what you're buying before committing.
If its from a friend or someone you know, you're immediately in a better position as its easy to see the phone and hopefully you can trust them.
If it at any point however seems too good to be true, it probably is.
The price on the site for a new iPhone 4 is currently £429 for 8GB so the question to ask is why are they selling it for half of that. It may be because they just have no need of it and aren't looking for lots of money and want a quick sale, or they may have got water damage, been unable to charge it, and are trying to get rid of it for as much as possible. (I had a water damaged 3GS and if someone had offered me £200 for it I would have laughed at them!)
I would ask for a serial number so you can use the support section of Apple's website to check about warranty, etc.
Perhaps a picture of all surfaces to show any scuffs or marks.
And ask if it is locked to a network. Its very easy to unlock if you are techno-minded but you can be charged to do it at a lot of places. (This is not a huge issue for the most part.)
Other than that it's mostly common sense, if you want to go as far as ask for a video showing it working fine, then that could work. ButI would say, if it doesn't feel right at any point, just don't bother, save for a little longe,r and buy a newer one from a reputable source. Not to say this person isn't completely honest at all. But it's better to be safe and wait a little than get a broken phone and be down £200. I hope that helped, maybe at least to confirm what you might have thought.
Also, it's important to know if the phone is GSM or CDMA (which will affect what plans you can buy), and which carrier it's locked to -- if any. This is especially important with iOS6, which can only be factory unlocked.