When's the iPhone 6 coming out?

And will it be a good time for me to sell my previous iPhone 5s? Someone's offering me £400 for it. Is that good?

No idea!

Analysts say that the iPhone 6 will be launched on July 2014!

No one knows other then guessing so why ask.

Likely September. As far as how much to sell your iPhone for look on eBay to get an average & price based on that