@rafiqul3 , is it possible that the eBay seller, after selling it to you, then reported it as lost or stolen and when EE had a look, discovered this and blacklisted it and this is why the eBay seller is not responding?
The eBay seller wrote, he got it from EE upgrade, the phone cannot be sold for the first six months, and so that is probably why it has been blacklisted, as EE will not unlock the phone until the six months are up. Sorry, but I would suggest contacting EBay and the seller, who had no right to sell the phone in the first place, as it is less than six months old and still belongs to EE. I believe there are other routes you can take to get your money back, but not sure and hopefully someone else will come up with some ideas. Always be careful where and who you buy from, when an item is so expensive.
To contact EE customer service dial 150 from your mobile phone or ring customer service for free using Skype or another phone: +44 800 079 8586 or +44 800 956 6000.
