The first sale proves someone was persuaded; the second may prove something was delivered.

A first purchase measures your description of the thing. A second measures the thing. Persuasion can carry someone across the initial decision without any help from the product itself, which is why early numbers say more about how you talk than about what you made. Repeat custom is harder to fake, because by then the buyer has the experience and no longer needs your account of it.

Wherever you are counting responses, separate the people who came once from the people who came back. If the second group is small, the honest question is about the thing rather than about the message.