The Clearinghouse offers a variety of Gear along with information.
When buying an item, the price in the Clearinghouse is the base price minus any applicable Perks, plus 15% when playing on Hard.
When selling items, the price is one quarter of the base price for each item, except for Ammo ($20/clip for standard; $50/clip for special). Intel, Gadget Upgrades, and equipped weapons or armor cannot be sold.
Four enemies will drop a 'trophy' weapon when defeated; these can be sold to the Clearinghouse, but then disappear and cannot be re-purchased: