ECPC is an authorized distributor for the cells they offer, and only sell cells that are authorized by the manufacturer for distribution in North America. These are absolutely, positively grade A cells that will meet all aspects of the cell data sheet and are supported by ECPC as the distributor, i.e., they handle your warranty.
I do not know if @OverkillSolar fits the above category or just buys (and possibly tests) and sells commodity cells.