Refers to the party selling something, often in contrast with 買い手 (buyer). Common in business and market contexts.
The seller's and buyer's terms didn't match.
この市場では売り手が有利だ。
Sellers have the advantage in this market.
He became the seller in this real estate deal.
The seller and buyer negotiated over the price.
The contract was delayed due to circumstances on the seller's side.