There are 2 kinds of Facebook groups local ones and then special interest groups. We are lucky in our area there are 18 local groups that we list and sell on. It is just like having a yard sale online instead of your yard. List the item and once someone wants it make arrangements for pick up.
The other type of online selling groups are special interest groups to join and sell on. There are groups for many vintage items like Christmas, Pyrex, Toys and many more. We also have sold many items on these group. We list the item with a price plus shipping. So then someone wants the item we would get their zip code and give them a shipping price and then send an invoice.
Selling on Facebook has helped improve our sales you should try it.