Here's trick you can use if you ever have to rely on someone else's scale to make sure it is accurate:

Weigh a nickel (the coint) on the scale by itself. It should weigh exactly 5 grams. If it weighs more, the person's scale has been tampered with to sell short bags. Either that, or it's broken.

If the nickel weighs less than 5 grams make sure you buy from whoever uses that scale to weigh the bags and keep your mouth shut! The scale weighs light, meaning you get heavier sacks.