In e-commerce or any business for that matter, it is very hard to get any sales without any marketing. And it’s very hard to get money to pay for marketing without any income so I would say both are equally important. But, I would probably give a slight edge to sales. The reason is because without sales to generate revenue, there won’t be anything left to market because you will go out of business.
Marketing is very important though because it helps you generate new sales, provides feedback on customer preferences, helps develop ideas for potentially new products and it generates new customers.