FWIW, I ordered something from Amazon last night that was listed as both being available directly from Amazon and also from Jeg's.

When I started to check out after having selected the item directly from Amazon (same price), they added tax. Then I checked again and saw that Jeg's expected delivery date for the same item was earlier, so I switched to the Jeg's-listed item strictly to get it sooner. When I checked out w/ the Jeg's purchase via Amazon, the purchase did not collect tax.