Just blocks that you can buy and sell for the same price in the shop. These could be any blocks, but ideally they would be unobtainable in survival and have textures reflecting their worth. The idea would be for
a. shops could be automated with hoppers and stuff so transactions could happen between online players and offline players, and
b. just to have "tangible" money.
The first point is the most important point, these don't really need to be blocks and I would be happy with any way to sell things to players while I am offline or not attentive to my shop.