A seller is someone who sells goods or services.
The term can also refer to an item or product that is being sold. For example, this item is always a good seller means that the product consistently sells well.
In some contexts, “seller” is used as a shortened form for a selling race, which is a type of horse race where the horses are all for sale.
Source: Collins English Dictionary — seller
This concise definition shows that "seller" can mean both the person who sells and the item being sold, along with a specific use in horse racing terminology.