I've seen a similar situation before. A friend bought a set of wheels/tires from a guy that had them on a Civic. The tires supposedly had around 1200 miles on them. We picked the wheels up at about 8:45 one night, then went straight to the shop to put them on. All of the wheels/tires were the same size so we didn't care which ones to put where on the car. Once we put them on and checked the air pressure (which is something I assume you checked, right?) we went out for a test drive. His car was pulling to the right. We couldn't figure out what was wrong. A few days later, someone else suggested we check the tread depth. Apparently the guy that had them before had done many burnouts with them, and while it was not enough to visibly make the tires look different, once we inspected them, the tread on two of them was considerably less than on the other two (the two he had on the front we had installed on the right side).
Without seeing your car, it's hard to say exactly what's wrong, but when using used tires, check the air pressure and tread depth. If those check out okay, it's time to get your alignment checked.