I done 5 KCBS contests last year and seen someone at each event to be what I considered "vending", which to me means selling. They all appeared to be former or current competitiors on the circuit. I don't think they are entered in the same contest they are vending, not sure though didn't ask. I have read about the "BBQ Bucks" on here but haven't seen that anywhere.
I always give anyway lots of meat and had people offer me money and would not take it. I don't consider this vending. I have been told by vistors that they had to pay other competitors for anything but I still don't consider that vending. Just my opinion. I have also met several teams that compete, own a restaurant and a catering company or any combination of the above. One team from Arkansas has the 2 wives stay home and run the restaurant and make all the money for the 2 men to travel the country competing on the circuit. (I really liked this idea!!!)
I would assume some contests has rules along these lines about vending but never look into it.