Facebook recently introduced the sale function in Facebook groups, but I think the Graph API is not available to use this function. If available, tell how to use.
Relevant images are attached here.
This is an api graphic request for a group feed: 
In this image you can see that the products are classified in different ways. Like their price, contact details. 
So the user can post messages manually on facebook: 
And this is the only product feed : 
EDIT: Here is the information of one message received using the api chart and the request mentioned in the first pic
{ "data": [ { "id": "230059773786620_695552487237344", "from": { "id": "809217099193795", "name": "Sukanta Roga" }, "to": { "data": [ { "name": "Buy n sell @NIT silchar", "id": "230059773786620" } ] }, "message": "200 NS for sell ₹75,000 - Contact no. 09436157708 The condition of the vehicle is very good. It has runs 11000 Kms. The price tag is negotiable. All the documents regarding registration are clear. Please contact PG hostel for more details.", "picture": "https://scontent.xx.fbcdn.net/hphotos-xpa1/v/t1.0-9/s130x130/10250295_802608569854648_5409368019938464401_n.jpg?oh=be63633e7a684e65c64d6cc13b513456&oe=562C9779", "link": "https://www.facebook.com/photo.php?fbid=802608569854648&set=gm.695552487237344&type=1", "icon": "https://www.facebook.com/images/icons/photo.gif", "actions": [ { "name": "Comment", "link": "https://www.facebook.com/230059773786620/posts/695552487237344" }, { "name": "Like", "link": "https://www.facebook.com/230059773786620/posts/695552487237344" }, { "name": "Share", "link": "https://www.facebook.com/230059773786620/posts/695552487237344" }, { "name": "Create Group Chat", "link": "/groups/230059773786620/" } ], "privacy": { "value": "", "description": "", "friends": "", "allow": "", "deny": "" }, "type": "photo", "status_type": "added_photos", "object_id": "802608569854648", "created_time": "2015-06-22T07:15:14+0000", "updated_time": "2015-06-22T07:15:14+0000", "is_hidden": false, "is_expired": false, "likes": { "data": [ { "id": "856694037754846", "name": "Roshan Prasad" } ], "paging": { "cursors": { "after": "ODU2Njk0MDM3NzU0ODQ2", "before": "ODU2Njk0MDM3NzU0ODQ2" } } } },