I have been thinking about doing some maths to calculate my market size. It would be something like the union/intersection of the set of all wanted book titles (that ReRead customers request) and the set of all book titles available in charity shops.
To create this market size model, we need data.
For the set of wanted book titles, this might be easy - we could maybe go directly to GoodReads and pull the data from public want-to-read lists.
For the set of book titles available in charity shops, this could potentially be modelled by looking at book popularity and volumes sold etc in the past. These books are more likely to be floating around and being donated by people. We could also look to apply some sort of weighting for 'irrelevance' or 'declining popularity/interest' - I assume this would also prompt people to donate the book.
Usually for unions of independent events, you can do P(A) × P(B), but the titles in these sets will not be independent? A very popular book that has sold 10s of millions of copies is both more likely to be in a charity shop and more likely to be on someone's wish list. So I may have to do some thinking about this. Maybe something like P(A | Title)...? Blog to be finished later..