Question

He sold ___ the old car and bought a new one

Answer 1

What to do? this is a right sentence

Answer 2

He sold off his old car and bought a new one.

Explanation:

A preposition is a function that is linked to the pronouns, nouns. Prepositions are used in ‘phrasal verbs’. Prepositions are information to any ‘noun or verb’ and signify various ‘relationships’ such as time, location, and position.  

Examples of prepositions are: with, about, of, for, since, from to name some. In this sentence up has been used as the preposition. ‘Sold off’ is a phrasal verb which means to dispose.