So, I have an iPad and I'm trying to look up the serial number through https://selfsolve.apple.com/agreementWarrantyDynamic.do but I keep getting this "We're sorry, but this serial number is not valid. Please check your information and try again." Does anyone know why? It was a previous demo unit, but I have fully restored it to factory setting and I am not having any issues with it. Problem is, I have another iPad that I wish to sell, but I am scared this one may give me issues? What do you guys think? Thanks in advance!