1) go to the seller, he greets you and asks you to show him the goods.
2) go to the boot.
3) with a choice of weapons Panel will appear.
4) press "W" and "S" to select a weapon.
5) press the "left Shift" (personally I have fast running key is configured on it) in order to buy the selected weapon.
6) press "Enter" to exit, after which the seller says goodbye to you all.