You'll be lucky if you even get 8k for it.
I sold my 1 year old Xperia U (9/10) for 7k, that too, back in February.
15k is a ripoff price for Xperia U.
Give him 21 and tell him to give you check warranty of at least 3 days. Check the phone thoroughly.
Discharge it completely to 0%, if it doesn't charge or if the white light blinks, tell him to give you your money back. This happens to very few though, but still do it, just in case.