Online e-commerce is without-a-doubt a good way to make money... if you can compete with the already established players; eBay, Craigslist, Etsy (amongst the many others). With all of that choice, how many times have you found yourself wanting to find a particular item near you?
You find something only to discover it's not even in the same state as you and the shipping is exorbitant/Paypal fees. The non-close proximity means more people see the item you want, which means less chance of you buying said item. While this is good for the seller, it equates to a lot of added complexity. What if you could just buy an item near you and go around and pick it up? No postage costs, no handling by the seller and an easy sale?
An application (most likely a mobile application for Android and iPhone) would allow you to browse products for sale a la Craigslist only within a set amount of miles/kilometres from you. This means you have a better chance of buying something you see because you have to be close or you don't see it.
Think of the mobile app as a localised classifieds platform, a bit of Craiglists with a little hint of Foursquare and Tinder thrown in. Add in badges and unlocks, a points system and you've gamified localised shopping.
I haven't really researched any existing solutions. This is something the bigger players like Craigslist, eBay, Etsy or any other classifieds/second hand store could implement. If you got the market share first, then you'd most likely be bought out before they'd compete with you.
Keep in mind e-commerce is highly competitive and this wouldn't be a walk in the park. Could be cool if you incorporate the gamified elements into the mobile app and only targeted mobile.