The number of online auction marketplaces available to purchase bulk lots of used iPhones and other mobile devices is growing rapidly. B-Stock Solutions appears to be the clear leader in the space, operating 6 or 7 such marketplaces. The run liquidation sites for Gamestop, Waste Manangement, Alltech Wholesale, eRecycling, Early Upgrade, MTM (Mobile Tech Management) and some sellers use BStockSupply.com to move these devices.
This has created an unprecedented opportunity for small buyers to acquire this inventory for their own business. For the most part, as long as you are a business with a resale certificate you can get an account to bid in these marketplaces.
You can find links to them all at the B-Stock Sourcing Network page.