Use Google Analytics to track sales?

I have a client that needs to be able to generate a link for an affiliate and then track any sales that came from that link. The shopping cart is Ecwid. I have setup Event tracking using Google Analytics but that doesn’t show me if a sale was made or not. I am open to suggestions and I also know it can be accomplished by passing a cookie. Can anybody help me with this? A quote or any help would be greatly appreciated!

I am assuming your client is generating the traffic, and sending via affiliate link the visitor to the affiliate’s site directly where the sale get’s processed?
So you need to track if a sale is processed on the 3rd party website?