Riding a wave of public outrage over credit-card practices, 7-Eleven Inc. wants to show merchants are victims of the industry, too. The convenience store chain announced a petition this week to give small businesses more power to negotiate the fees they must pay whenever a customer uses a credit or debit card. 7-Eleven said more than 6,000 of its franchisees plan to collect 1 million customer signatures to deliver to Congress this fall.