It might be useful on first thought but to be honest, there are also some pros and cons..

First of all, it would be troublesome if it goes so many ways around. Try to imagine people here are purchasing things because they are cheap. It might be hard for them to pay something extra to someone that they doesnt know in the first place for their service..

Besides, there is also a risk and more extra charges for posting items. It goes 2 ways now which is from Seller --> Middleman and from Middleman --> Buyer. That doubles the risk of posting fragile item and not to forget, paying EXTRA again as well..

Time is also a factor now which leads to you needing to wait longer to get the item.

Safest thus far is still COD. It is always known to have risk buying items online. There is also risk even buying directly from shop. Its up to the buyer whether they know what to do or not in the first place. Still better to look closely and research before purchasing anything..

Well, what they are suggesting is, when a deal is done, the buyer will send his money to the middleman and the seller will also send the stuff to the middleman.

In this sense, the middleman will have both the money and stuff. Therefore, when both reaches the middleman's hand, he will then proceed to send the item to the buyer and the money to the seller.

It makes sense to be safe in one part but does not if you think about it closely. Good monitoring will be needed, cost a lot more, takes more time and doubles the security risk. If the middleman would run away, 2 party will be at risk..