I use USPS because the prices are relatively low and things get shipped out in a timely manner. They're free tracking is also very convenient when selling on eBay and their offices are everywhere making it fairly convenient to drop off packages to be shipped off. Usually when I'm buying from a seller I don't really care which service they offer me as long as the items get to me in the expected amount of time. But I prefer free shipping when shopping online so any seller that does give that option, that's the shipping method I normally go with. The things that I buy are usually not need immediately so waiting isn't all that of a problem for me. But when I ship, I always use USPS.