I agree with you about it needing to be a certified scale to be legal. The scale probably needs to be checked by the bureau of weights and measures once a year as well.
Way back I have bought seed a number of times from hardware stores and they just weighed it in the certified scale and dumped it into a paper bag, no labels or test info. I think you are probably right about legally needing the test results and certification but I think there is a lot of seed sold without it. Nowadays I buy my seed by the ton. (pallet of 40, 50 pound bags, actually)
In a small scale operation you can probably just weigh it on a good scale and get away with it but you won't really be legal. Another option might be to sell it in a plastic bucket and price the bucket without saying it is "x weight" and just saying it is about 5 pounds ie, one bucket of seed $ 12.95, approximately 5 pounds.