Iphone 5c eBay bad esn

Here's the thing. I live in the UK and I want to buy an iPhone 5c but I don't want to give nearly £500 for one because I'm a foreign student and I earn less than £500 a mont so that's not an option. I saw on ebay that they sell from USA iphones with bad esn very cheap like £250 and it sounds really good for me because they are sealed and new. But what exactly is the bad esn? I reseached it but didn't quite got it. If I buy one verison bad esn iphone 5c will it work in UK? Will it ask for activation? Does the activation sim cards they sell on ebay actually work? Will I need to unlock it?

The phone will not work in the UK.

The quick answer is that a bad esn means the phone will not work, anywhere, not ever.

A bad esn means the phone was lost or stolen (or shop lifted) or the owner defaulted on his phone contract thus owes his carrier money. The phone is "blacklisted" and can't be used as a phone, it is a useless item only good for parts.

You will notice that bad esn phone are much cheaper than working phones.