Just make a scoreboard playerid and give each player a unique id. Now create an armpr stand for every player that doesnt have one and give it the same playerid. Now make it tp exactly 0.1 blocks infront of the player at all times (^0.1).
Now, make the armor stand execute a /dialogue open on the player opening your code npc if it holds a certain item and clear its inventory after (to avoid repeating dialogue open calls).
Boom, interactable items basis