If you are interested in buying secondary market mobile devices in bulk, there are a number of marketplaces in which you can do it. The best marketplaces are run by B-Stock Solutions for various companies. The marketplaces are all auction-based, which is great since it allows you to name your price and possibly win. They are well laid out, easy to navigate, and you are buying directly from the companies selling them (not from B-Stock or another middleman).
Here are some sites you should check out:
Alltech Wholesale: cexchange.bstocksolutions.com
Early Upgrade: earlyupgrade.bstocksolutions.com