You might want to reconsider your business model. Apple only gives a year warranty (possibly extended an additional year if the buyer paid for it). Apple service out of warranty is slow and expensive.
So once you buy a phone, pay apple $$$ to fix it, and find someone who wants to buy a used iPhone (probably a couple of years out of date and possibly so old it won't run the latest iOS and apps), I don't see much profit.
It would be better if you could be selective about buying phones that you could fix (replacing cracked screens is relatively straightforward).
But this is not a legal issue. Apple is free to do decline to repair phones from you.