NAIROBI, Kenya—On safari for the rare African tech unicorn, Goldman Sachs Group Inc. and other big-name investors think they have spotted a winner in Jumia, the continent’s aspiring Alibaba.

The online retailer, which launched in 2012 in Nigeria with the backing of German startup fund Rocket Internet SE, is Africa’s biggest e-commerce platform selling goods and services to millions of customers. It has raised more than $700 million from investors, including global telecom operators and was valued at $1.2 billion—qualifying as a so-called unicorn—during its last fundraising round in 2016.

Jumia has expanded from four to 14 countries. Gross sales last year reached 507 million euros ($597 million), up 42% from the year before. Its vendor network, from Cape Town, South Africa, to Casablanca, Morocco, includes up to 70,000 businesses offering their goods and services online.

Jumia’s growth story outlines the scale of the challenge for African online retail. Faced with poor internet connections and tight bank lending for vendors and consumers, the startup has had to build from scratch much of the economic infrastructure within which to operate.

“Nowhere else in the world is it more difficult to shop than in Africa,” said Sacha Poignonnec, Jumia’s co-founder and chief executive.

Jumia executives have set up credit lines, offering loans to vetted vendors from the company’s balance sheet. They also hold training workshops on basic accounting and stock-keeping.

Just 0.5% of all retail on the continent takes place online, according to Boston Consulting Group research, compared with about 15% in China—home to Alibaba Group Holding Ltd. —and 5% in India. But companies like Jumia are changing the economic landscape for many African entrepreneurs.