Can I swipe someone else's card and sign his name?
Yes, usually you don't know if the place where you swipe your card is yours, unless it's a credit card with a photo. Most places in the north don't check signatures very much. As said downstairs, nobody asked about signing a Yao Ming. But if you go to the south, especially Guangdong, they check the cashier carefully, let alone go to Hong Kong, Macao and abroad.