The Game Trading Zone (gametz.com), has been around in one form or another since 1997. I was originally called "Used Game Trading Zone" (UGTZ). In that time, it has grown rather organically. It's a website for people who trade video games. Movies, music and books are also supported, but computer and video games are the primary medium that is traded. Most of the people who use GameTZ are gamers.
People create accounts where they can list items they have available for trade and items they want. Every account has a page where their lists are displayed along with other information. Listings can be searched to find people to trade with. There is a matching system that will automatically find people who have what you want and want what you have. The site is focused on trading, but buying and selling are also common.
There is a offer system that helps people come to trade agreements. GameTZ does not make the trades for you. Instead it provides a number of tools to help people make trade with each other, directly (P2P, if you will). Once an offer is agreed upon, a trade record is created. The offer and trade help both traders negotiate and track the deal. A log of all activity is kept. Each trader reports when they send and receive. Once they receive, they also give the other trader a rating.
Reputation is a key component of the site. Traders who have made a lot of trades and received good ratings, get various levels of star icons next to their username. There are a 100s of regular traders on GameTZ who have traded with more than 100 other traders.
There are risks to direct trading like this. GameTZ has a system for reporting "Bad Traders". GameTZ does not take responsibility for losses, though the moderators do try to help resolve disputes. Using common sense and safe trading practices allows the savvy to avoid most problems. Overall, the value gained from direct trading tends to greatly outweigh the risks. Many of GameTZ's regular users complain they have more games than they have time to play.
The site has never achieved great success. Membership remains in the low few thousands. It has gone through various transitions, including a period where the main trading features were subscriber-only. But, that didn't work out. Currently, and for the foreseeable future, use of GameTZ trading system is entirely free. Some people still subscribe to support the site and/or get access to some extra features (like turning off ads). But, subscribing is not required and most traders are not subscribed.
GameTZ was created and is maintained by Bill Marrs who is active on the site and continues to directly respond to most users. Part of the reason the site has grown organically is the tradition it has of encouraging suggestions and implementing them.