" Some items have been stacked on a cart "
This is a sentence in the present perfect and in the passive voice. You use "have" because "items" is plural. It refers to the action of stacking:
- Somebody has stacked some items on a cart -> Some items have been stacked on a cart.
I think " Some items is stacked on a cart " has the same meaning .
That sentence is not grammatical. You should use "are" because "items," as I said, is plural:
- Some items are stacked on a cart.
This refers not to the action of stacking but to the state of being stacked. "Stacked" there is like an adjective.
- Some items have been stacked on a cart and, as a result, they are now stacked on a cart.