Hey there! I play on a SMP server and have an iron shop myself. If you go mining for your shop's stock then you should consider giving it out 32 for a diamond. Though if you have a iron farm (like me), you should sell it for a stack per diamond, especially if other players don't have a good source of iron.
Well, if you do have an iron farm try and sell depending on the rates of the farm. Super fast? Sell it for cheap. Fairly slow? Sell it for less.
I have a farm, but it is on bedrock so rates are slow, but come update it should be a lot better, atm i get 80-100 ignots per hour. Come update that should be about 10× faster so im thinking maybe a stack for a diamond atm and come update 2 stacks for a diamond. Idk still a bit unsure. But thank you for your advice its helped.