Inseam length is more useful than height when it comes to frame size. Has the OP fitted a bike before or is this the first time? If the OP has a bike that is set up right, then trying to get in the same riding position on the new bike is the way to go.

I doubt that the seller steered you wrong on frame size if you gave him your dimensions. They sell a lot of bikes and know something about fitting. Long legs at 5 ft 9 suggests that the 55 is probably the right size. This is useful: http://www.coloradocyclist.com/bikefit.