OfferUp is a platform where you can post classified ads and reach millions of potential customers in seconds and buy almost any product near your area at auction prices.
Functions:
The application offers a reputation system where you can hear the opinions of other users who have done business with the sellers and buyers. Some people even give away their stuff for free.
How to use:
Take photos of the product(s) you're selling and specify whether you will send the product to the buyer via a courier or if you prefer to meet in a public place for the exchange.