Headinclouds - you MUST start in the country where you purchased the ticket from. Sure you can buy the ticket from an agency via the internet and pay by creditcard. But you still must go to that country to start your journey.

If you don't the airlines can have the autority to decline you borading in USA when they check your passport and don't see a stamp in say, Bogota, where you might have bought your ticket from.

Better be wise than lose a ticket!