Myself any my partner found an easy way to trade ammo and weapons... Granted it's somewhat ruthless, but at this point in the game it is necessary:
1) In shelter, select the weapon(s) and ammo(s) you wish to trade.
2) Enter a lobby
3) Find the closest exit and move there quickly as possible
4) Kill your friend (or have your friend kill you) and retrieve the weapon(s)/ammo(s) being traded *THIS WILL MARK THE ASSAILANT AS A THREAT*
5) Leave the area as quickly as possible, as you or your partner now have a giant red target on you in the map.
Fun side fact: Killing your friend also counts as killing a survivor for all challenges.
