In game you will go to the menu where it will show the weaopns. If you havnt bought it then they will say purchase but if you do buy them then you just press them and it will spawn nearby. It will be like you just bought it from the shopkeeper guy, on the gorund and you walk on it to pick it up. I dont know about multiplayer but i think it would just be the same becasue its LAN Just have your friend pick it up.