A VN friend bought her Iphone 6 for "4.5 M" new

Well can tell you that it was not new....

Could have been that 4.5M was only the initial payment to receive the phone.  In exchange, your friend would be locked in a 2-year plan with the provider and extra payment would be added to the phone bill every month.

That's how it is in the States.  Many people paid only $100-$150 for a brand new phone, but with higher phone bill for 24 months afterwards.  Same as buying a car with a low down payment.

When I traded in my iPhone 5 for iPhone 7 last year, T-Mobile offered me the same thing -- $99 up front and $35 added on to the phone bill each month.  I didn't want to be in debt for a phone even though I really liked its camera, so daughter bought it for me and paid the whole thing at once ($750 for 128 GB).

BTW, the smallest iPhone 6 (16 GB, new) is less than $300 now in the States, so your friend's deal is perhaps not too difficult to find.  The price reflects the value, however, since 16 GB is not enough memory for anything.

Apple no longer direct sells the 6 in the US, only the 6S.  This may be a recent change as a result of the new release of 8 and X. 

Amazon US has unlocked 6 phones with 16 gb for $279 refurbished and $379 new so your friend's price of 4.5 mil VND is objectively too low.  There could be two things at play.  1) The phone is a fake.  This is usually easy to detect as the fakes run Android.  2)  The phone was stolen, probably in the US and brought to VN to be sold.  This is more common than you might think.

As Ciambella said 16 gb is really too small if you intend to load any movies but as most Vietnamese that I have observed use iPhones primarily for photos, Facebook, and maybe some music, 16 gb may be enough.

@Chris: There are many videos on YouTube to help differential between a real iPhone 6/6S and a fake one, if your friend wants to know (but it's a fait accompli, so maybe she doesn't.)

@THIGV:  I don't have movies or videos on the phone, but I've never thought I could live with 16 GB (Good Lord, I've become spoiled!)  So, I'm checking my phone's data right now. 

There are 47 apps, 3975 songs (I do listen to them all, believe it or not, since they're all very carefully selected), 2814 photos (from recent trips; the old ones are in cloud and external HD), 1467 ebooks, and 304 audiobooks (deleted and added weekly so both numbers are fluctuated.)   Add whatever else I can't see at a glance and whatever else Apple used for their stuff, I have 67 left of 128 GB. 

It's excessive, I think.  My life is actually very simple, but the stuff in my devices are probably not.

Well, the battery of my old 4S went on strike and would not charge for over 20 hours. Then it was back at 100%. // My cloud is almost full with photos... And memory seems to be like displacement in ICEs   

A Thinkpad X230 from Ebay might be an alternative, as it has a sim card slot? The display is bigger   

BTW, I was suckered twice, buying "unlocked" NOKIA phones (830 + 928). Guess what? Both are locked and wifi only.

Has anyone imported electronics through Ebay's int'l shipping program? It's so tempting to just order stuff overseas...