There are many people selling postcards on the streets a lot of them are disabled in some way and yes it is hard to not feel sorry when you see how hard the people work in Vietnam especially in the north where they do everything manually.
Unique Suggestions We did buy postcards but only 1 set each in every place simply beracuse they are sold in packs of 10 for $US1but they are all of the 1 place. When we were approcahed we just told them we already had some thank you they seemed pretty ok with this and many stayed to chat with us and ask where we were from. Generally we found the people to be very friendly and not a pushy as we had expected them to be.
On the streets of Hanoi there are hundreds of people selling various things. The most common thing is children selling postcards or chewinggum. The children can be very cute and friendly, but oh my! The price they want you to pay for the postcards! If you just arrived to Hanoi, are tired and don't really know the money yet, these little ones can really cheat you!
Unique Suggestions Be friendly but determined!