Can I trade in my iphone 6 for a iphone 6 plus?

Can I trade in my iphone 6 for a iphone 6 plus?

There's loads of secondhand tech shops and mobile phone outlets which stock used examples of both phones.

So yes, they'll be happy to buy in your unwanted iPhone 6 and sell you an iPhone 6+. Just don't expect to get anything like the same price you can get for your iPhone 6 by selling it privately as the shop has to buy it for a low enough price to make a profit and provide a limited warranty.