Startup Idea : BUY SELL SWAP - everything to anything

Selling on kijiji now days is very hard with spam and a lot of dealerships and such.... I want to make an app that a buyer can shop around and check for a certain item in a category and purchase in person or online such as eBay and while the transaction on certain items (such as a cellphone) the buyer can pay for shipping cost. All that incorporated into one app. And obviously you cannot buy a car online through this app. That is in person ... But other things ... Hope someone can help with this :) !- Praveen JS