I must tell you the only thing the Iphone wins on is the size and quality of the screen IMO. The SE X1 just seems to be that much better and also has the Tomtom installed, makes a big difference.
Well let's try and set a precedent for the sale of a "2nd hand" 16GB 3G iPhone
A quick Google Search brings up some Gumtree and Junkmail ads.
Gumtree : R4000.00 That's if the original price was R7500
JunkMail : R5500.00 seems to be a constant, though it seems like the same two people posting wanted and "for sale" ads.
So hopefully that is some kind of guideline for ya
EDIT: Though - one cannot discern if those are serious/legit sellers