My experience with milk bottles is that if you find people in the area from where the bottle came from, they're willing to pay top dollar for it. Usually eb*y is a good bet because most collectors will keep their eyes on the auctions for particular objects.
As for milks, I'm just getting into them myself, seeing how much they can bring...Typically, the older bottles are embossed, the newer ones are "painted." The older ones tend to be round, the newer ones square.