I think it may be state law, but also depends upon whether the store is aware if you have poultry or not. I wonder if the local TSC is going to worry if you are buying large bags of poultry feed all the time and want a few different breed chicks. The 6-minimum is more to discourage the impulse buyer of a child's pet.
I picked up two RIRs the other year at our local feed /hardware store. They were close in age to those in my brooder and I didn't have any of that breed. No problem-- but then they know me.
If you are 17, you may be looking for employment soon. Doesn't hurt to mention that you know about chickens!