Well the resale is a good point, although you could argue that for anything that's expensive. Like I could buy a Mercedes coupe instead of my Mini Cooper and justify it because I can sell it for more. I obviously could, but I'm still out of that up front cost, and I don't get any of it back until I sell the car. And with a $500 iphone, you'd have to sell it for $400 to be in the same ballpark as an android. Maybe that's a reasonable amount for an iphone, I don't know. I've never looked at used ones.
Then there's always the possibility of it getting damaged, which I assume will depreciate it, or lost, or stolen, which would negate the resale value completely.
I'm sure there are scores of people who buy and resell phones regularly, but for me, personally, I'm just too clumsy/careless to make it work. Plus I like to tinker, which I probably wouldn't feel comfortable doing on an iphone. With my luck, I'd end up with a $500 brick.