The thing is, in order for someone to buy an iPhone 3G, they had to sign up for a 2 year commitment with AT&T in order to get the subsidized $199 price.
If they’re selling their iPhone 3G prior to the end of that commitment, then they’re still on the hook for meeting the obligations of the 2 year term, as well as for paying back the subsidy on the phone (like $300).
Of course, they might try selling the thing at a high enough price to make up the hit to their wallet, but then it’s not really gonna be a good deal for you.
So, if you’re looking to buy a used iPhone 3G (less than two months since their initial release), you better be very wary.