Learn Finnish: Voiko psykologinen temppu saada ihmiset ostamaan enemmän? - Can a psychological trick get people to buy more?

This is the core of the "causing an action" structure. When you use the verb saada to express that someone is caused or convinced to do something, it requires the object (here, ihmiset) followed by the third infinitive in the illative case (-maan/-mään). So, saada ihmiset ostamaan literally translates to "to get people into buying."

Doesn't saada usually mean "to receive"? How does it mean "to make" or "to get" someone to do something here?

You're right that saada usually means "to get" or "to receive" a physical thing. However, Finnish uses this exact same verb in the saada + object + -maan structure to mean "to cause to happen." In English, this translates very cleanly to the causative "to get someone to do something."

Why is ihmiset in its basic plural form instead of the partitive ihmisiä?

In this sentence, ihmiset is the total object of the verb saada. Because the question implies a successfully completed result (the trick actually succeeds in making them buy), a total object is used. For plural words, the total object looks identical to the basic nominative plural (ihmiset). If the sentence was about trying to get them to buy without a guaranteed result, you might see the partitive ihmisiä.

Could I use laittaa instead of in this sentence?