For reasons beyond comprehension, Apple iPhones are frequently stolen and offered for sale on the streets of many cities, especially in Pham Ngu Lao in HCM's backpacker slum.
In future, before buying iThingies from dubious sources, pop on over to Apples Apple Activation Lock Status Check, enter the serial number or IMEI of the potentially 'hot' goods and see if the offer is good.