I think a lot of the success from facebook buy/sell groups is that the items will actually show up in people's newsfeeds once they have joined a regional/topic

I think a lot of the success from facebook buy/sell groups is that the items will actually show up in people's newsfeeds once they have joined a regional/topic group for trading. This probably raises awareness of second hand items available in general. If the marketplace is going to be a separate part of facebook, It might not be as engaging.