I can tell you it's not paying for more marketing if you already have good traffic! That's like a lion trying to hunt a gazelle by just running it down until it collapses. To increase conversion rate you have to do something that directly affects sales.
Running is needed by the lion to get the gazelle in range so he can go for the kill. Many shop owners keep tweaking and tweaking marketing but don't do anything to close the sale, this is like a lion who is just running and running and running.
At some point, the lion has to go in for the kill. You get people to buy your product by closing the sale. Until the customer puts the product in their cart and pays for the item nothing happens.
You have to put the effort to close the sale and when you can do it in a repeatable way, you can start having a predictable sales forecast. A good online pitchman / product demonstrator closes the sale for you.