Final answer:
The verb 'vender' should be conjugated as 'venden' to match the subject 'Ellos' (they) when talking about selling books in Spanish.
Step-by-step explanation:
Ellos __venden__ libros.
The verb 'vender' is conjugated in the third person plural form to match the subject 'Ellos', which means 'they' in English. The correct ending for this conjugation is '-en'. Therefore, the correct form of the verb is 'venden'.
For example:
- Ellos venden libros en la tienda. (They sell books in the store.)
- Ellos venden libros usados en línea. (They sell used books online.)
Learn more about Conjugating verbs in Spanish