Start off selling accessories. Make sure you provide tracking for everything. Sell slowly... make sure you can be reached when ebay tries to call. Mark up on accessories is bigger than mark up on phones.....
When you list an item, use real pictures, try to show proof of purchase and or receipts. Spread out the sales.... keep a back up ready to go... or 10
At some point ebay or Paypal might ask you for invoices, What do you intend to show them, considering that there is only 1 Master Distributor for them? and that you have to be an authorized At&T reseller in order to buy them. I am Quite sure that Ebay Knows ALL This and More.
I can sell MY iPhone on ebay, no problem. You don't have to be a ATT reseller to buy them either...... For sale with service, sure. But what's stopping you from walking into a store and buying 10 at full retail?