Coupons: Is there a group or site to see/exchange/gift coupons?

So I collect games, but I'm not interested in badges/levels. So I rarely get coupons, but I came to understand that every time you level-up you should get a coupon.

Some graceful SG members have gifted me coupons, and I realised this is a great way to buy games I don't have so I'd like to know: how are coupons generated? Is it randomly or according to your wishlist?

Also is there a group or place where you can exchange or gift coupons?

Thanks.