Where do you sell? When I look in ebay the iPhone 4's selling are selling in the 180-220 range. There are few buyers art the gazelle rate.
I sell privately to a person I know, on Craigslist or on eBay...any of those would infinitely be better than Gazelle (and the other like websites that offer you "guaranteed value").
iPhone 4's are selling for $180-$220...Gazelle is offering $140 for the iPhone 4, so again, you're missing $40-$80 profit even on that 3 year old iPhone.
As far as the iPhone 5 goes...there are TONS of them selling on eBay right now (most with 5+ bidders in a bidding war over them), and my local CL also has dozens of listings for unlocked iPhone 5's. Hell...even the lowly Sprint iPhone 5's arnt as low as Gazelle is offering on them, they are in the $375-$425 range on eBay (WITH bidders).
Just for example's sake, here's one of the first ones that pops up on eBay, this was without even scrolling down to see the others in the range...(30 bidders...also notice it is damaged cosmetically)...
https://rover.ebay.com/rover/1/711-...1299493218?pt=Cell_Phones&hash=item3a829df562
...using Gazelle is essentially handing them free money.