I totally disagree with this statement.
I've bought many many IP4 from Craigslist and maybe twice I thought they were stolen.
You can get one from CL for $400 or under easily.
Well, maybe you are from a more trustworthy part of the country. But out of the four iPhone 4's that I looked at around Mar. of this year, I got to the point where I was going to buy the end of the conversation went something like this:
"Are you the original owner?"
- Yes.
"Do you have the original box?"
- No.
"Do you have a receipt from AT&T or Apple or the contract or any paperwork at all of the details?"
- No.
"Would you be willing to call Apple or AT&T and get a copy?"
- No.
"Can you offer any proof that you purchased the phone?"
- No.
"Ok, well, thanks. I think I'll keep looking"
- Wait. I'll knock $50 off the price.
And if you think I'm making this up or you want to disagree that this is what happened... then ok. But it did. The above transcript was more or less word for word two of them. Another one was a guy who constantly changed his mind on selling it - ok, he might have been legit - he said he had a receipt but no box - but he was frustrating. He agreed to the deal, we agreed to meet. He wanted cash. We were an hour before we were supposed to meet and he calls and raises the price by $50 to $450. I said no, and he calls back a day later and offers it again at $400. Then we are supposed to meet at noon at Starbucks and he calls 20 minutes before we meet and tells me he changed his mind. Then he calls back the next day and offers it again. And I told him he has to be kidding. The last guy was legit - he worked for Amgen, had the original box, said he bought a Verizon iPhone, etc. - but he changed his mind several times too and he strung me along for almost two weeks. So I gave up on him too.
If you've had better luck than this, but I think 3 out of the 4 phones that I tried to buy up here in Northern Colorado in one month (Mar. 2011) on Criagslist were dubious and even the one that was probably legit was really annoying.