I thought about creating a marketplace where people can exchange coupons they purchased from sites like Groupon, but haven't used yet. Do you think this is a good idea? anyone implemented this yet in the USA?
http://www.lifesta.com/ http://www.dealigee.com/ http://www.dealsgoround.com/ http://couprecoup.com/
Now if you're talking about non-Groupon type coupons as well such as regular retail coupons, there are a few of those as well including the major bargain aggregating communities.
Some are definitely successful businesses. Whether or not it's a good idea is up to you (it's going to be subjective for everyone). Just because you have an idea for a social network doesn't mean you'll be a Facebook. You could be one of the other thousands of unknown networks. It's all about how you execute.