So how will this work? will you convert USD into some coins and use that to buy stuff similar to Amazon and then also have the option to share with a friend like facebook?
USD, Euro, other coins.. But the usage of coins would be for peer to peer transactions, so for example I'd purchase your used watch. Also you could use those same coins to promote your sale post: let's say you published your sale post, but didn't get as many offers as you would've liked: you can use a bit of your coins to promote that post, so it will be visible to more people.
Other uses of the coins will be tipping: the Roomba you'd like to sell isn't working, but is giving you an error. You're out of warranty, and just want to know if it can get a cheap fix or not. You ask for help, and when someone provides you with help, you can tip them with the coins.
A few other uses as well.
You could share any posts you like with your friends: "Just bought a new phone", "My phone is not charging, anyone able to help?" "Check how I tuned my mining rig", etc. In addition to your friends, with tags other people will see those posts as well and be able to comment, like, etc.