I recently ran across three broad hatchets at a flea market. All had blades about 10" long, and seemed to be in good shape. The handles were tight, the blades were semi sharp, but as I sighted down the flat edge of the blade there was a slight curve of 1/6" to 1/8". Should there be this curvature in the blade? What else should I look for? How much should I pay for a broad hatchet?

There are quite a few patterns of broad axes, but generally yes the blade should curve. If you lay the axe flat on a table the blade should rock.

Think of it as giant gouge, this keeps the corners from digging in. Check prices on Ebay for a comparison, and for pictures of examples. Generally, the good one fetch the higher dollars, Generally.

Eric