For several months or years at a time, the local suppliers may be more expensive than big box stores.
This is what I have found for basic residential supplies like NM cable, PVC conduit etc. at first I figured it was because we are a small outfit so I asked a friend at a larger company to compare and he said the box stores were also cheaper.
He then proceed to buy them out of NM cable.
After that they set a limit of rolls per customer.
NM cable is often a 'loss leader' where box sores will sell it at loss and advertise a great price to get you in the store, then make it all back when you buy all the other overpriced stuff.
Loadcenters at the box stores around me all have an aluminum buss where as the wholesale house panels have a mostly have a tin plated copper buss. The bus bars look identical. I am old school and prefer the plated copper buss.