I think it's a little high. Womens framed bikes (be they step-through or mixte style) are never worth as much as comperable mens bikes because in general womens bikes never got ridden as hard and there are a lot more of them still around. I think $100-150 would be more reasonable in this area (Greater DC) but maybe the market is different there. You might repost this in Classic and Vintage as those guys buy and sell bikes like this all day long and might be able to give a better idea of what these things are selling for.
I'd at least try to get the seller to drop the price by what a set of new tires would cost.
Nice looking bike though. I just noticed Bikedirect put out some new mixte city bikes (single and 3 speeds) and I wish my wife would ride one if I got it for her.