Online marketplace Jet.com Inc. has almost no revenue, years of likely losses in its future and a strategy that includes underpricing mighty Amazon.com Inc. on millions of items. Jet also has perhaps the highest valuation ever among e-commerce startups before their official launch.

That is no contradiction in Silicon Valley, where investors keep pouring money into audacious business experiments filled with big-splash potential. Jet is the buzziest e-commerce arrival of the current boom, with $225 million in capital raised...