They are passive gerund clauses. "The most complaints received in any sector relate to people being sold faulty cars..." = "The most complaints received in any sector relate to the selling of faulty cars (to people) "people being sold faulty cars..." acts as a noun phrase in this sentence. It is part of the same sentence. The main verb in the sentence is "relate". The verb form in any gerund phrase is always -ing.

Yes, it is passive and also related to relative clauses! :) You can read the sentence like this: "the most complaints received in any sector relate to people [who are/were] being sold faulty cars or being tricked..." You could also write, "...relate to people who have/had been sold...", and now you see that writing the sentence is becoming a little unwieldy. Which tense should you use? The good thing about this kind of clause is that you don't need to worry about the tense. Just put that extra information right after the thing being mentioned.

This is actually the present progressive of the verb "to be" in an elliptical construction. It is as follows, "the most complaints received in any sector relate to people [who are] being sold faulty cars or [who are] being tricked..." Upon further reflection, it is a form of passive voice. Your second sentence is not using the infinitive either, "to" is a preposition. It should read, "relating to people who are being sold..." Depending on the remainder of the sentence "relating" would be a gerund or participle.