Lets say you park a domain, what prevents you from just linking to that site in your forum sig etc and getting traffic/revenue from there? Do domain parkers only pay for type in traffic? Do they pay based on clicks or impressions?
Depending on the parking company you decide to use.. There TOS may not permit signature links in forums. Im pretty sure SEDO does not accept that as a valuable source of traffic. Pay Per Click.
Parking companies only allow 'organic' traffic. This means that you cannot promote your parked domain whatsoever. The only traffic allowed is what comes in naturally through search engines and what people type into their address bar.
My parked domains are promoted daily... You just have to promote them in a way that gets people to type the address in the address bar. eg: my t-shirts.. ads on my truck.