dont really agree with the above if you are thinking seriously about growing would probably purchase seeds.
potency can vary greatly from the many strains that are available and you may not end up with anything better by using bag seed even if it was grown in perfect conditions, often commercial strains are high yielding but may not be very potent and growing conditions are not really going to improve it much.
the fact that the bag had seed in it to begin with is also somewhat of a concern either the grower didnt care if the plants got pollinated or they got pollinated on accident possibly by a hermaphodite wich will very likely pass on that trait if the bag seed is used.