Customer(s), i.e. the buyer, purchaser, consumer, household, i.e the person or other who approves and consumes the products(goods)/services of the seller/producer, creating the supply and demand circular flow.
Worker(s), the person(s) and/or their substitutes (machines) offering the services/work in the business, producing and/or selling goods or services.
In amateur activities (hobbies etc) the consumer/buyer is usually the same as the worker.
Exchange of satisfaction, interdependency: buyer, <--> seller
The buyer/consumer is the cause of existence of the producer/seller and
The producer/seller is the cause of existence of the buyer/consumer.
All the above form the holotes of business, i.e. different parts joined together forming an entity different from its parts.