You can't legally guarantee it (even the GDPR, which is like the top of the line in terms of privacy regulations, is still a joke due to the lack of enforcement

You can't legally guarantee it (even the GDPR, which is like the top of the line in terms of privacy regulations, is still a joke due to the lack of enforcement) but you can make yourself look more trustworthy by having a business model that does not rely on stealing data.