Over on Swappa an unlocked 128GB iPhone 6 has an average selling price of around $470, with the last one sold fetching $430. Based on this small amount of information, I'd say a fair market price to ask would be between $440 and $500. It would really depend on how quickly you want to sell it. The prices are not as high for carrier locked versions.I have been trying to sell my 128gb iPhone 6 with 13 cellphone case and I have not had any luck. Maybe I am asking for too much? What would be a fair asking price?
Don't sell on Swappa - only because of PayPal aka RapePal. Those greedy bastards took $24 on a $600 sale of my 6S. NEVER AGAIN!!!
Better off selling on Craig's List locally. Just remember to meet either inside a carrier store or at your local police station.
Compared to eBay, $24 is pretty low. Imagine having to pay eBay $60 on top of Paypal's fees! Which would be $84 between the 2.