I collect measuring tools and have several of those hand stamped carpenter's squares. I try to pick them up for 5-10.00, but I have paid 35-40 for larger or interesting ones.
Look in the corner area for a maker's name, which can add to the value. There were several square makers in Vermont in the 1830s to 1850s. They merged into Eagle Square Co. in 1859 (EAIA, 1999). If you find a name (George, Essex, Whipple, Beach, Hawes, Loomis were some of them), then I can look it up in the EAIA Directory of American Toolmaker's and give you a date range.
There are also quite a few unmarked hand stamped squares out there.
I am going to a flea market in Lewisburg, WV at the Fairgrounds this weekend or I might try that Luray swap meet.