Most people who show interest in what you are selling won’t buy from you right now. Some of them, however, might in the future.
When their behavior indicates they are not going to buy from you now, get them to commit to doing it in the future. They are more likely to think about such a future commitment in abstract terms such as “I should do this” (if they are convinced they should, because of their views about themselves), and would commit more quickly. When you ask them to commit now, they will approach it in a more practical way like “I can’t buy this now, I don’t have time for it.” This is especially true if they know that what you are selling is good for them but for some reason, it is difficult for them to do it (like eating healthy or exercising.)
The best list to have is of paying customers. This will make sure you have only customers on your main list.