When someone enters a Fb ad for a specific item,what are the chances he will also buy something else

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When a customer enters a Facebook/google ad for a specific item, what are the chances from 0% to 100% that he will purchase something else from the store? (a store like aliexpress which has everything).

I'm asking because I am debating whether I should finish the whole shop before I start with the adfrom 0% to 100s and it will take me a few days or shall i start with the ads first and add all the categories with timefrom 0% to 100