The RIBs for Sale forum is as stated, one way comms. Interestingly, the North American version is two way - and sometimes they really get their undercrackers in a knot. Put it this way, can you imagine the comments from the usual suspects if they COULD reply!?!
Quite a lot of sellers are on RIBnet purely to advertise a RIB for free and forget to check their PMs. If there is a seller who is advertising a "sold" boat or failing to answer PM, report it and one of the Mods will remove it in due course.