I think the difference between the dot-com boom and today is that back in the '90s, companies that were essentially speculative ideas with no proven market were receiving high valuations, whereas today companies that are targeting a proven market are receiving high valuations because they need to raise (and spend) a lot of money in order to successfully penetrate the market and grow quickly enough to achieve the economies of scale they need to compete effectively.

Marc Lore's a good bet for this particular market. He's already built a successful e-commerce company from scratch and sold it to Amazon. It appears that his plan for Jet.com is to tailor prices for each individual shopper in real-time, effectively targeting a profit margin of 0%. People in the US are familiar with the Costco/Sam's Club retail model (i.e. where you pay a membership fee to access discounted prices), and real-time, personalised pricing is incredibly powerful. You get to price off an individually-defined demand curve for each customer, and if people get familiar/comfortable with discounts that are applied to a shopping basket, rather than individual items, you reduce the competitive impact of a low price advertised by your competitor.

Also, it looks like Jet.com plans to operate as a marketplace as well as a traditional e-commerce company: https://developer.jet.com/getting-started