Maybe you could try MercadoLibre? It's basically the eBay of South America. Looks like the going rate for an iPhone 6 Plus (64GB) is 17,000 ARS. That's about US$1900 at the official exchange rate, but before you get any big ideas I'd STRONGLY suggest doing two things:

1. Find out how sellers on MercadoLibre are paid, and much more importantly, how the proceeds can be converted into USD. (Actually, you should probably start with finding out *if* they can be converted. They might require you to have an account at a local bank.) If you have to use their own means of "currency exchange," your net income might be reduced considerably.

2. Make sure you're not going to get into trouble bringing in a bunch of iPhones to sell. I can speak from personal experience that the entire area of Argentinean import/export is rife with land mines; I was almost arrested once because I was missing the correct paperwork for some modern art I was departing the country with.

Also, IIRC eBay *itself* has launched operations in Central and South America, so you could try them as well. (I don't think their regional sites are accessible except from a local IP address, however.)