My experience with duty-free stores located on the US-Canada border has been on the Canadian side. In the three Canadian stores I've been to (two in Quebec and one in British Columbia) the only exit from the duty-free store leads directly to the US port of entry, so that any shopper who has visited the store has to cross the border.
However, on the US side duty-free stores don't seem to be set up this way -- the duty-free stores in Derby Line, VT and Highgate Springs, VT, for instance, are located on public roads not on the border and a driver could easily visit the store and then not proceed to Canada.
Given that, how do these US stores ensure that travelers who've purchased goods there take them out of the country (in this case, into Canada)?