It's not easy earning a decent living in the wasteland, so inevitably you learn to cut corners wherever possible - craft items instead of buying them, sneakily murder people carrying bigger guns than you, or just outright steal vendors' best items from under their very noses. To do the latter: Open a trade window with the trader who owns the item you want to obtain. Put one of your less important items up for trade - a single bullet, a piece of junk, whatever - so its value appears in the small box at the bottom-centre of the screen. Highlight the item you want to steal in the vendor's inventory, then press X and Square at the same time on PS4 (X and A on XBO), and the item should appear in your inventory at the same time as the 'Confirm trade' box appears on the screen. You can cancel the trade, because you've already got the item you need. Pressing the buttons simultaneously can be tricky and might not work first time. If it fails, try pressing Square just before X on the PS4 (X before A on XBO). Keep trying this trick (and cancelling the trade each time you get asked) and after a couple of attempts you should have whatever it is you sought.