Your argument says that no one should EVER buy a used iPhone.
No. My argument is that there are risks involved in buying a used phone.
AT&T started unlocking phones nearly a year ago, and the problem of hacked and jailbroken phones in the gray market shows no signs of slowing. Eventually, I'm sure it will, but not until the supply of 2nd hand 3G's and 3GS's starts to dry up. The vast majority of those in the pipeline were probably already there before AT&T started offering unlocking, meaning they most likely got hacked quite a while ago.
No, I don't know where the OP got it. But, if it was from ebay or craigslist, or Amazon, or some other source that has become a haven for liars, cheats, and thieves, the odds were high from the beginning that the phone was misrepresended as legitimately unlocked.
Buying a used phone from a trusted source such as someone you know, is a world away from buying one from an unknown 3rd party.
You should trust someone selling a used iPhone just about as far as you would trust someone selling a used car.