"Unit" is often associated with the individual item being sold, while "pieces" is more about the overall quantity.
"Unit" typically refers to the individual item being sold. It refers to the finished product that lands in the customer's hand. When you list a product on Amazon, you might specify the price per unit.
For example, if we're talking about a set of plant hangers, one unit would be one set of these hangers.
"Pieces" is a more general term referring to the total quantity of items. It doesn't necessarily indicate the individual selling unit.
For instance, if you're selling a pack of 5 pens, the number of pieces in that pack is 5. The term "pieces" is often used to describe the overall quantity.