What does it mean to kill the salt seller?

This is a joke or complaint when eating a big meal. The complete statement is "kill the salt seller." If the salt seller is killed, he can eat salt for free and put salt in the pot, which is generous, so the food is salty.

A joke with the same meaning is "Marry a salt seller". If you are in love with a salt seller, you can eat salt for free, and it is easy to generously put salt in your meals to make them salty.