[ot1138]

As pretty much any professional trader or quant knows, there are degrees of efficiency. Ex-US markets are significantly less efficient than the US. Some US securities are less efficient than others. Certain instruments and exchanges are less efficient than others.

Markets become more efficient as a result of entities who make them so. It is possible to make a very good living by being one of these entities.

[gmd63]

And it follows that making a market inefficient through nefarious tactics allows said entities to continue to make a very good living.

[bjornsing]

At what time scales would you say these inefficiencies are easiest to make a good living from?

[financetechbro]

I think OP is referring to brokers and market makers, whose role is to help match buyers and sellers and to maintain liquidity in certain securities, respectively. In which case the time scale would be small (you don’t want to be caught holding something for too long if you’re just a middle man)