Learn German: Wer kauft an diesem Stand ein? - Who is shopping at this stall?

It belongs to the verb einkaufen (to shop), which is a separable verb. In standard sentences and questions, the conjugated part of the verb (kauft) takes the second position, and the separable prefix (ein) jumps to the very end.

Why do we use diesem (dative) here instead of diesen (accusative)?

The preposition an is a two-way preposition, meaning it can take either the accusative or the dative case. Because we are talking about a fixed location (where is the shopping happening?) rather than a movement toward a destination, we use the dative. Since Stand (stall or stand) is a masculine noun, the demonstrative word becomes diesem.

Does the question word Wer always use the singular form of the verb?

Yes! Even if you expect the answer to be a group of people, Wer (who) always acts as a third-person singular subject, just like er, sie, or es. That is why we use kauft instead of the plural kaufen.

Since this is a question, why doesn't the sentence start with the verb?

Because this is a W-question that starts with a specific question word. The question word Wer takes the first position, and the conjugated verb (kauft) safely stays in the second position. Verbs only move to the first position for yes/no questions.