Go on ebay and search for below market value phones.
Find phones that the only problem with the phone is a broken screen. You can fix broken screens on phones really easily once you know what you're doing.
Make the sure the phones are not locked either.
Do not buy phones that have a bad ESN number (CDMA Phones) or an EMI number for GSM phones.
Once you get the phones and fix them then you can resell the fixed cell phones on ebay and make a profit.