As it turns out, this question has several possible answers. American English, for example, refers to collective nouns differently from British English; Americans are more likely to treat them as single units. And there’s a lot of room for personal choice on the matter too (but think about who you’ll be debating with once you’ve made your mind up!).
It seems that the most important factor, when making a decision about using singular or plural verbs with collective nouns, is consistency. For instance, if you agreed with me in the example above and took the plural option, you’d have to be consistent with this choice. It would be confusing to say ‘the family agree but it won’t sell immediately’.
Luckily, there are a couple of easy ways to solve the problem. If you truly can’t decide on whether the collective noun is singular or plural, you can recruit additional words to clear up any possible confusion.