Nothing.

By benefiting others in the form of sending merit you are essentially helping them with ranking up, and the forum ( by weeding out/making the difference between good and bad posts).

Unless ofcourse you make a deal with someone that you trade your merit for their merit, however not only is that against the rules, it also won't work in the longterm, due to you not being a merit source and only receiving back 0.5 sMerit/1m...