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Just don't go to a Carrier. Verizon is still selling the iPhone 5s for $50 more than the SE. Doesn't make sense.

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eBay might be a good place to look.

On eBay I sold my 5S for about £125 and it even had dent marks on it. Most likely it be second hand but I'm guessing you can get one for about $100-150+ in the US if you look hard enough.
 
Pretty much your best bet will be a used iPhone 5s from eBay, Swappa, etc. The iPhone SE was already priced so low that no one can really sell even a 2.5 year old iPhone 5s for less than what the SE costs.
 
Looking for a new but discounted 5S. Anywhere I can purchase?
I bought my 5s from eBay, it's refurbished and came in new box with all new contents and the phone looks brand new, not a mark on it. I am really happy with it.

Edit: forgot to add it cost £150
 
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