I may have misunderstood your question, but as far as I'm aware anyone can put a property up for sale on these websites.
I used an estate agent to sell my home, who put it onto these websites for potential buyers to view, and it sold within a few months.
I've bought properties through Rightmove and Zoopla; they link you to the estate agent and dialogue can open up.
I find these sites the best tool for property buying.