The concept is technically interesting, but I don't want my e-mail address to be harvested by third parties and sold for marketing purposes. I am not a lawyer,

The concept is technically interesting, but I don't want my e-mail address to be harvested by third parties and sold for marketing purposes. I am not a lawyer, but in the EU (which is where I assume you are located) data privacy regulations require the user to consent to the use of their private data. The sites you collect the information from supposedly have permission to process private information by means of their privacy policy and relationship with the user; which as a third party, you don't have. One could argue the data is already public; but since the user can not consent – you have no relation to the user – you are going to find yourself in muddy legal waters, especially trying to base a business on that data.