To be legal, you will have to weigh it on a certified scale, (in most states). You could perhaps weigh it on a kitchen scale with a 1 to 20 pound capacity.
Perhaps it would be legal to sell it by the quart. Perhaps...
Some stores around here use a self-serve dip-out-of-the-barrel system, but I think they must weigh it at checkout. It may not be legal to sell seed without the label indicating test date, variety, and percent germination.