Is it allowed to offer here a boosting service in wow for some arena/rbg rating boosts for ingame gold?

Service vs igg is perfectly fine (ofc everything on the same server and realm).
Just remember that mediators cant help You with that services, they cant hold igg, fg only.

Not advised imo unless a mediator can hold.

most of the eu traders are pretty legit..and not many do afford to buy high stuffs anyway