It sounds as though you have a bad battery. I have noticed that my iPod and iPhone do charge more quickly when plugged into the car. It seems as though you are able to give the battery a quick boost but the capacity is depleted. If the guy you bought it from is unwilling to repair it or you don't want to go through the hassel of dealing with him, you may want to tak eth opportunity to pickup a battery kit from here and try the repair yourself.