Stacking Items?

New guy here...
Is there a way to stack common items in my pack? Like berries? Someone told me I can't but I look at player run stalls and I see common items in large stacks in the stalls. Is it only in stalls that we are allowed to stack common items?
Is there a way to stack common items in my pack? Like berries? Someone told me I can't but I look at player run stalls and I see common items in large stacks in the stalls. Is it only in stalls that we are allowed to stack common items?