Each item is tracked one of two ways, and the choice decides what you can do with it later.
Use serial units when
The item has a serial number, a warranty, or a price high enough that you want to know exactly which one you sold: air conditioners, compressors, refrigerators, generators.
Use quantity batches when
Items are interchangeable and sold by count or length: gas, copper pipe, remotes, brackets. These carry a weighted-average cost, optional expiry, and can never go negative.
If unsure
If a customer might come back and say this one, use serials. Otherwise use quantities.