Premonetary Barter Economies in the 21st C.

Person A wants Good X. Person B has Good X. Person B declares that she will barter Good X for Goods Y or Z of undisclosed types. Person A doesn't have those goods but could acquire them. Should Person A acquire several samples of Goods Y & Z for purposes of trading with Person B and similar individuals? If only there was some medium that would allow Person A to exchange with Person B in a straightforward fashion . . . .