with a Duldung you cannot apply for a Gewerbeerlaubnis (trade licence); it is not sufficient.
You can act as middle person only if you are employed by s.o.to do this (and this employment has been authorised by the Ausländerbehörde). It is possible that you can have a forein employer. However, as soon as you act on your own, i.e. for several foreign companies (as an Agent) you will need a "Gewerbeerlaubnis", which you will not get at this stage (see above).