Not always true, indeed many of the best agents neither push nor use any devious methods. Selling oneself limits the vendor to selling to their own language speakers, thus for English speakers without French some of the best potential buyers are lost. The commission level is entirely a matter for negotiation between the agent and the seller, contrary to popular belief, who will pay it, the buyer or the seller is negotiable and part of the mandat de vente, which is a contract that must clearly state the percentage level of commission and who is paying it, otherwise it is invalid.
French courts have judged that where the seller deals directly with a buyer who has been introduced to them by their agent, or uses a proxy to find out the address by using an agent and thus send the eventual buyer negotiate without, in order to avoid paying the commission then it is usually the vendor who is obliged to pay damages and interests to the agent, thus more than the original fees in many cases. Bear in mind as well that whilst some buyers do not like paying the agent’s fees, it is usually to their advantage to do so since that reduces the stamp duty on the property. However, most agents' fees are paid by vendors with commission included in the advertised prices.
Many of the sites and links you have put up are actually put together by agencies to gain credence against competing agencies but are well disguised to look like either independent or official information. They lull people into a sense of false security and people are often ill advised to take many of them seriously. If one agency says look at what our competitors are doing, implies that they are illegal practices that they would never do, then uses those sites to prove their point then people should simply walk away because the chances are very high that the agency has its own scams.
Yes, using leboncoin and such means works but only if people are confident about the business with notaires and other things like the energy and polluting materials type surveys. A good agent will always help or even arrange those things, then when the sale is completed there are things like electricity, water and sometimes gas, buying in other types of fuel, the telephone and a whole mountain of other things that can be done. Any agent who gets the completion over and walks away is not doing their job thoroughly.
Whole rows of links like this are not helpful really, especially if preceded by the first two paragraphs which are in the first instance inaccurate and in the second potentially leading people into trouble and extra costs because of misleading and ultimately dangerous advice.
I am not an agent, by the way. I am well informed by an agent who is successful and does not rip people off or walk away once a sale is pushed through, indeed the pushing seldom works given the average notaire snail pace.