Learn Spanish (Latin America): ¿Quién compra esa casa?

Quién means "who" when you are asking about a single person, or when you aren't sure how many people are involved. If you specifically expected a group of people to be buying the house, you would use the plural form: ¿Quiénes compran esa casa?

How do we know which verb ending to use after quién?

When you ask a question with quién, you conjugate the following verb as if you were talking about "he" or "she" (él or ella). Since comprar (to buy) becomes compra for he/she, we use compra here.

The English translation says "is buying". Why doesn't the Spanish sentence use a word for "is"?

In Spanish, the simple present tense like compra is very flexible. It can mean "buys", "does buy", or "is buying". You don't usually need a separate word for "is" to talk about an action happening right now.

Why is the word for "that" esa and not ese?

Words for "this" and "that" must match the gender of the noun they describe. Because casa (house) is a feminine noun, we use the feminine word for "that", which is esa. If you were talking about a masculine noun like a book (libro), you would use ese libro.