I've got something you might be interested in. I need a quick decision — this thing's hot.
On the “upgrade agents” screen between missions, the agency sometimes has the option to purchase an item from Monst3r. Most of the items Monst3r offers are available from some Nanofabricators, but Monst3r sells them for a 20% discount compared to their nominal values. Other items he offers can still be acquired by other means; no items are exclusively available from Monst3r.
Monst3r has something for sale at some particular scripted times, and otherwise randomly. Any item offered by Monst3r can be purchased only before entering the next mission; it will not still be available afterwards.
Whether or not there is anything to buy from him, the agency also has the option to sell items, weapons or uninstalled augments to Monst3r, for half of their nominal value. Anything sold to Monst3r cannot be bought back from him. However, the same things may become randomly available to buy from Monst3r later.
Perhaps unexpectedly, trading with Monst3r outside of missions works no differently even when Monst3r is on your team as an agent, and even when Monst3r has been lost in the field; this is not remarked upon in the game.