You can only sell stuff at Kumoricon if you are one of the people with an official booth.
That is the official line. If you set up a table on the street or in the park to peddle your wares, you'll have to deal with the City of Portland.
If you really want to sell stuff I suppose you could do so out of your hotel room. Maybe hand out business cards on the sly to advertise, but even then you wouldn't be allowed to advertise anything officially, with fliers or the like.
My advice, don't try it. Too many things can go wrong.
