No, we don't need 'was'. Here's why: In the phrase 'Very little of the remaining stock sold', we are actually using the verb 'sell' as an intransitive verb, with the stock as the subject of the clause. Normally, 'sell' is a transitive verb, as in 'The store sold very little of the remaining stock'. However, this sentence uses a different form of 'sell': also known as an ergative form. Other examples of ergative uses of verbs are 'This pasta cooks in five minutes' or 'This paragraph reads well'. We often use these forms in more professional contexts: in the case of your sentence, it's the kind of phrase that would be used in a business context. Of course, you could add 'was', but this would change the grammar of the sentence. If you said ''Very little of the remaining stock was sold', this would be a passive form of the usual transitive sense of 'sell'. That would be equally good in terms of grammar, but it would sound less 'businessy'. Your sentence is better left as it is. I hope that makes sense.

Not strange at all! The sentence is perfectly correct.

It's strange, because this sentence was initially given in the exercise.

Yes, use 'was sold' here.