How do you go about starting an E-commerce business like Shopee, Lazada, or even something like Shopback?

Traditionally speaking, an online marketplace is one of the least capital intensive business model (usually asset-free or asset-light at the beginning). However, the downside is that it has a low barrier of entry.

To figure out how much money you need to start an online marketplace, you’ll first need to identify and understand the problems you’re trying to solve, what makes your value propositions unique.

From there, you’ll be able to define your go-to-market strategy by gauging how strong your value propositions are (marketing, sales, management, execution, product or tech superiority).

When it comes to your go-to-market strategy, are you planning to grow at all cost? This means acquiring suppliers (merchants) and demand (customers) at a loss-making rate to carve out significant market share quickly. If that's the case, you're going to need a whole lot more money to survive the bloodshed.
(E.g. None of the companies you've quoted is in the green. They’re still burning millions of cash year on year)

Or do you plan to focus and invest your resources in building a fantastic product (the marketplace platform) while using tech and product superiority as the main differentiator?

More often than not, having a fantastic product (your marketplace platform and its network effect) increases the chances of winning over the market share in the long run while maintaining a decent unit of economics.

Quoting Mark Zuckerberg’s advice to the Airbnb founders when Wimdu (+90 mil funded, dominated the market share in Europe) came knocking for an acquisition, “Don’t buy. Whoever has the best product will win.”
Airbnb is now a profitable multi-million company, while Wimdu has ceased its operations in 2018.

In today’s context, the $50k that you've spent years saving can be expended by a company in a matter of minutes.

Hope this helps!

TBVH, but that's just my own opinion, is that the whole e-commerce platform space is rather crowded, and with the industry already consolidating nicely to 3-4 players (L, Shopee, Q10, Amz), there might not be much space here in SG to start.

However, if you look at more frontier markets like VN or MM, the Internet space may not be that mature. If you have some nativity in those countries, replicating a matured business model in SG and starting it there might give you a bit more breathing space than in the arms race already happening here.

ShopBack and Shopee/LZ have very different business models and value propositions to the merchants they serve. The thing to ask yourself is where do your core strengths lie? Performance marketing? Logistics? Customer experience? And then build your platform around that.

Money-wise, it depends on how spartan you want your post-founder lifestyle to be. The day of the launch is glamorous, after that it's eating grass. Some VCs recommend anywhere from 12-24 of your last drawn salary. My take is, if you can drastically adjust your lifestyle, you might survive with less than that before you raise a round.

Sources: bought and assembled ShopBack's first 10 IKEA tables