Brothers Mortimer and Randolph Duke explain to William that they're commodities brokers. They explain what commodities are and provide examples of them. They then mention how they earn money: through the commissions of their clients.

Raj trades Sheldon his limited edition Green-Lantern lantern in exchange for Sheldon's service as a wingman.

Penny ruins the cushion of Sheldon's seat on the couch and he brought it to Penny's dry cleaner. However, he doesn't trust her dry cleaner because he also makes keys.

Describes the role of Athens as a trading city in the Mediterranean, famous for its olive oil and innovations in pottery style and decoration.

Susan doesn't have the money to buy an item she wants, so she negotiates with the store owner, offering her items she has, in return for the item she wants.