Think of it as a North Korean Best Buy — a shop stuffed with refrigerators, karaoke machines, laptop computers and flat-screen TVs. But with a square footage closer to a typical American 7-Eleven than a big-box store, the Pothonggang Information Technology Center has to be selective about what products it puts on its shelves. So it’s telling that nearly one whole wall of the shop was crammed last week with solar panels, inverters and battery…