Hello, Please how do I know when to use "estan vendiendo" and "venden"

Hola Childalu

I imagine you're referring to this test question: 

Ellos ___ su casa. (They are selling their house.)

with correct answer "venden" (as opposed to están vendiendo).

Sometimes in Spanish we use the present tense (vendo, vendes, vende...) for actions that "are happening" at the moment of speaking, like this one. However in English, the most natural translation for this is the continuous present "are selling". It wouldn't be incorrect to say "Ellos están vendiendo su casa", but in this case, the hint is asking you to conjugate vender in the present, not the present continuous. 

I hope this clarified it.

Saludos