as one of my character is a clan fixer.. I think that the idea of being able to be killed while carring illegal items is bad.. I think that a fixer should have a nano. CONTACT MARKET. that way you can "order" things from the black market, or be able to find where the market is today.
for penalties - jail would be good, my be a zone that could be escaped from. like a mission field, you just have to find the way out. or have someone came in after you. I would not make it a a specific spot, like each town has their own prison. have the character that was caught arrive resurrect there. trying to sell an illegal item, would be caught with an item gets you in combat with nearby guards and/or shop keepers.. selling to players to close to guards will have guards attack.. have the radius for guard to notice vary, between ranks.
have fixers be able to communicate between clan and omni.. or have to go thru neutral fixers to get the others stuff..