Only difference between money and karma is that we can't easily trade karma. Modern money is not physical, it is just a figment of our imaginations. It has only the value we collectively give it.
Do we have to revisit the barter system where value is traded for value to once again realize the reason money was invented? Fish for tools works, until you want apples.
It doesn't matter whether something "intrinsically" has value (wood as fuel, apple as food) if you can trade it for something that does. As long as that remains true, karma remains an int and bank accounts remain buying power. Fin.
Anyway, for karma to be effective, all it has to do is have enough value to outweigh the (equally intangible) value of the satisfaction of posting something trollish.
It's not like a bank keeps a physical amount of cash equal to all account holders' deposits (if they did, they'd have nothing to lend). Instead all you do with your money is transfer it, or get paper vouchers for it in forms like checks, money orders, or federal reserve notes......
That people accept those for things that are physical doesn't change the fact that the money is in fact not a representation of anything physical.