Startup Idea : Want to buy

A bit like a reserve ebay. Problem: Many people put up what they are looking for or wanting to buy in fb groups. This is only limited to the groups you are in. Also when you search on google it only shows you the top ranked, or something that may have already sold. I want an app where people type in what they are looking for and with tags sellers can be notified if someone is looking for there product, they may otherwise have never be found.