Answer: Nosotros se las vendemos
This is a good example of the usage of se as an indirect complement, in order to substitute the indirect complement le or les.
What does this mean?
Let’s begin by explaining that the Indirect Complement of a sentence is the person, animal or thing that indirectly receives the action of the verb.
To identify it in a sentence, we must ask to the verb ¿a quién? (to whom?) Or ¿para quién? (for whom?)
In the case of the sentence Nosotros vendemos el pan a las personas (We sell bread to people) if we ask ourselves the following question:
¿a quiénes vendemos el pan?
(To whom do we sell the bread?)
The answer is:
A las personas
(To the people)
Being las personas (they) who indirectly receives the action, therefore the indirect complement of the sentence.
Now, the Indirect Object Pronouns replace a word or phrase, which in the sentence fulfills the function of indirect object.
They are written before the verb, when it is conjugated.
These pronouns are listed below:
1st person singular Yo: me
2nd person singular (informal) Tú: te
2nd person singular (formal) Usted: le
3rd person singular Él/Ella/eso/esa: le
1st person plural Nosotros: nos
2nd person plural Ustedes (In latinAmerica): les
2nd person plural Vosotros (In Spain): os
3rd person plural Ellos/Ellas: les
Then, returning to the sentence Nosotros vendemos el pan a las personas, knowing that the indirect object is las personas, who is the 3rd person plural Ellos/Ellas (they); the indirect object pronoun in Spanish is les.
In that way the initial sentence changes to:
Nosotros les vendemos el pan (a las personas)
We sell them bread (to people)
This is where the personal pronoun se comes in, as an Indirect Complement, and replacing le or les (indirect object pronouns), immediately followed by one of the following pronouns depending on the direct object (the part of the sentence that directly receives the action):
lo (singular masculine)
la (singular femenine)
los (plural masculine)
las (plural feminine)
In this case las is used because las personas is feminine and plural, therefore the sentence:
Nosotros vendemos el pan a las personas
Can be written as:
Nosotros se las vendemos
Where se and las are the two object pronouns.