Shadowrunners are penalized for selling stuff because they are usually trying to unload stolen goods in a hurry and off the books. They are penalized for buying stuff because they want rare and/or illegal stuff, sometimes in a hurry, and usually off the books.
For a fixer type, he would not be in the typical shadowrunner's role, but in the middleman role. People would unload cheap stolen goods to him, and would pay him a commission to broker deals to get them stuff. Sometimes there will be serendipity (someone unloads some SMGs taken from some security guards cheaply, and the local street gang, looking for heavier firepower, pays extra for them). A fixer, assuming he has the connections, will generally make money both ways. However, street cost modifiers can affect his bottom line (fake AK-97 knockoffs flooding the street, a rival dealer offering cheaper Kong chips, one guy controlling the influx of awakened botanicals being smuggled through the Cascades, etc.). Also, he will need to worry about rivals, people out to squeeze information out of him, syndicates wanting to bring an independent operator into their stable, and greedy punks who see a fixer as a tempting target for robbery.