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.
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.
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.