Just as we purchase water plants for our ponds every now and then, from time to time we also buy fishes. Koi are a particularly sensitive breed of fishes, so extreme care must be exercised in “transporting” them from their place of purchase to your pond.
The most “basic” of these precautions is to make sure that the Koi have been “double-bagged” (i.E., placed inside two layers of plastic bags), and that two or more “extra” plastic bags were provided for the “transportation”. Also, check whether the retailer did add “extra” oxygen from an oxygen cylinder. Do the same if you were doing the “selling”.