Neither is correct. This is subjuntive 3rd person. It translates as "let them buy". Polite imperative is 2nd person only
how do you differ between should and have to? or how can you say which one is correct?
This is a problem with the Beta version of the course. Just report it if it tells you you should have used the other one.
What is ambiguous about this is that this word also means "buy them"
Agreed. "Wanunue wale kuku pale walionenepa sana." -- Buy those chickens over there that are very fat.