I'm having trouble understand the difference between an unlocked phone bought from Apple for top dollar, and an unlocked phone from Verizon, also for top dollar. Since each is unlocked for international use, what's the difference?
In general, when people refer to "unlocked iPhones" in the US, they're referring to the GSM versions. Those come from Apple unlocked with no sort of activation with a US carrier required to use overseas. This makes it usable out of the box if someone wants to use the iPhone only with another country's carrier (though it may not be wise to buy a iPhone in a country other than the one in which the user lives).
The Verizon iPhone can have the GSM portion unlocked for use with carriers in other countries, but requires that you activate with Verizon before they will unlock the GSM portion. That often is impossible or at least impractical for users who do not live in the US.
That's the difference.
Regards.