Refers to a person or entity that puts items up for sale (e.g., at an auction, online marketplace) or for display (e.g., at an exhibition).
The seller at an auction must describe the item's condition in detail.
I registered as a seller on the flea market app.
出展者 is used for exhibitors at trade shows or exhibitions, focusing on display rather than direct sales. 出品者 can also mean exhibitor but often implies items are for sale.
Compound of 出品 (shuppin, 'exhibit; put on sale') + 者 (sha, 'person'). The exact historical derivation is straightforward; the word follows a common pattern for agent nouns.