commercially available product; off-the-shelf item
Refers to products that are sold on the general market, as opposed to custom-made, homemade, or specially ordered items.
This part is a commercially available product and can be easily obtained.
I compared the quality of store-bought and handmade items.
既製品 also means ready-made or off-the-shelf product, but emphasizes that it is pre-made rather than custom-ordered, while 市販品 emphasizes availability on the market.
Compound of 市販 (market availability) + 品 (product). The word is transparently derived from its components.