While I urge Canadians to use common sense to assess whether or not the GSP makes sense for a purchase they're considering, I doubt that a "boycott" of sellers using the program will change anything.
As I think I mentioned to you elsewhere, I believe the majority of sellers using the GSP are ones with little to no experience shipping out of the U.S. prior to adopting the program for their use. A boycott of GSP sellers is simply going to mean that their sales are likely going to remain unchanged from before they started using the GSP.
I would recommend sending a polite message to any seller using the GSP for an item in which you'd otherwise be interested, explaining why you won't purchase from them, how much you're being charged in shipping for the item (the seller likely won't know) and how the applicable import charges aren't often charged by Canada Border Services even though they're (mostly) applicable. This way, they get a sense of how much interest there is outside the U.S. for their items, at least. You may even find some who aren't even aware that their listings have the GSP applied to them.
