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Your idea for a screenshot maker, especially one focused on long page screenshots with quick save/copy/email options, falls into the 'Freemium' category. This means people generally like using tools like this, but convincing them to pay can be tricky. With 6 similar products already out there, the market isn't exactly wide open, meaning competition is present. Given the medium engagement (avg 10 comments) on similar products, there is definitely interest. You'll need to figure out how to stand out and offer enough additional value to encourage users to upgrade to a paid version, or find the right users who are willing to pay for this type of product. Think about who would be willing to pay a monthly fee for this or similar.