We need a trading system, then
Like this, you put your crosshair over a player and press T, then it requests a trade to them, if they press T it opens up trade. It shows his inventory, and your inventory side by side. It has two buttons (Trade and cancel) and it has a six by six grid of what you can put items and blocks in to trade. You can trade in stacks of 32, and if their inventory is full it doesn't allow the trade.
Edit: I will make up an example picture just to show