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.

4 star : 4 star weapons with a much higher chance of getting 5 star weapon.

5 star : Weapons of 5 star.

Will there be any UV's
Yes, but with a small rate of getting one.

Will I be able to cancel my trade?
No, so be careful of what you put.

Will there be any fee?
150 ce.

Feel free to ask me any questions!

EDIT

The weapons have limits on which tier you are.
For example if you're tier 1, you'll get a 1 star weapon.
If you're' tier 2, you get 1-3 star weapons.
Tier 3, you get 1-5 star weapons.