Iphone suddenly has no network, says "phone not allowed"

Well... you got scammed. You were the victim of a pretty common scam, in fact. Apple has nothing to do with blacklisting. Only the carriers do that. Return it and get your money back. If the seller refuses, file a claim with ebay/paypal.

What happens is someone orders a phone from their carrier. Sells it on eBay. Reports it to their carrier as not delivered/stolen in the post, and gets a replacement from their carrier who blacklists the original. They basically get a free phone and you're left holding the bag.

Next time, buy from an authorized reseller instead of eBay..

@Giulio, KiltedTim has identified the problem correctly. The phone has been reported stolen, and blacklisted by the original seller, usually a cellular network. There is no way to get the blacklist removed, the phone is stolen property and most likely the legal owner carrying the loss is a cellular network or an insurance company.

As KiltedTim says, you are the victim of fraud. Raise a case with eBay and PayPal to get your money back. There is no point calling or going to an Apple Store, they can’t clear a barred IMEI set by one of the phone networks, and because the phone is reported stolen the banning network will never release the ban. The phone is useless, a very pretty desk ornament but useless.

ProustGiulio wrote:

This sentence, coming from Apple Support (if true) doesn't convince me:

"Weirdly, when I called apple support, they are saying it is not blacklisted.......".

That's why I suggested the OP to drive to an Apple Store.

Apple has no way of determining that as they aren't involved in blacklisting and there is no universal blacklist they could be checking... I'd write that statement off to ignorance on the part of the support rep.

"My network can't help as I didn't get the handset through them"

I can't believe they won't help you. Talk to a chief technician, contact them directly. Insist.

Perhaps driving for an hour and have your iphone inspected, is not so dramatic. Apple Store employees will give you all the available options.
If you get no help, then yes, contact ebay and/or paypal.

We can do nothing else for you, sorry. We're just users like yourself.

Regards

Giulio

Why should the OPs carrier help? This has nothing to do with them. They can't remove the blacklist on the phone if another carrier blacklisted it as stolen. Even if it was blacklisted by the OPs carrier, the best they could hope for in bringing this to the carriers attention is that they won't be charged with receiving stolen property...

This sentence, coming from Apple Support (if true) doesn't convince me:

"Weirdly, when I called apple support, they are saying it is not blacklisted.......".

That's why I suggested the OP to drive to an Apple Store.

As far as carrier is concerned, they should be able to find out if that iphone is / was really blacklisted, and confirm that situation to the OP.

I don't have your or Tim's certainties.

Giulio