Looking at the title, it's not a vendor who trades weapons..
It's actually a machine trader..
Now, What is weapon trader?
It is a machine in which you put 3 weapons in exchange for one weapon.
The '3'means you put must be something of 1 star to 5 star
What will be the result?
The result is random, if you put in a 1star weapon, you're likely to get a weapon which is 1 star to 3 star. There will also be a chance of getting a 4-5 star weapon.
2 star : 2-3 star weapons with higher chance of getting 4-5 star.
3 star : 3 star weapons with higher chance of getting 4-5 star weapon.