How do I legally sell my email list asset, and do I need to sell the business along with it?

There are legal, regulatory and risk implications to retaining and handling funds as a marketplace. The good thing is with marketplace platforms becoming more mainstream, the payment solutions ( such as Stripe, BrainTree, WePay etc) have also evolved to support their unique needs/services provided. Depending upon your marketplace model you could choose to just transfer funds to the Seller minus your fees/commission OR hold onto funds (for a certain period) giving your users the ability to withdraw funds upon their choosing. The payment solutions you choose vary by the use case, and the solution providers take responsibility for legal, regulatory and risk implications. I have experience in payments for maketplaces and would be happy to share more feedback and suitable solutions over a call.

Cold emailing is just as bad for you and the recipient. Even if you have the perfect list, the attempt to sell in a cold email is rarely going to be effective. You're better off curating the list to the top prospects, find a mutual connection on LinkedIn or even just cold-invite them on LinkedIn,. Worst case scenario, send a 'permission pass' email where you simply gauge interest and let them know you won't be emailing them again if there's no interest. Keep it very short, non-commercial with just solid information/links to web, and an easy to reply yes/no answer.

It always starts with the list. You mention 'non opt-in' lists, which is certainly more troublesome than any domain name. Your best course of action is to get a new domain, re-confirm the lists you have (requiring the subscriber to click through in order to stay on), and only accept confirmed opt-in lists moving forward. It will decimate your mailing list in the short term, but long-term eliminate your deliverability concerns and improve ROI.

Non-disclosure agreements are very common. Know that a non-disclosure agreement goes both ways as well. A contract agreement should also protects you from the other party disclosing any trade secrets or know-hows that they may learn from working with you.

Your emails have a lot of room for improvement. You have to: 1) Give a 1 sentence intro 2) Get right to the point quickly in a clear and easy to understand way. Eliminate all the long winded and confusing language (i.e. everything in quotes: "business attitudes towards....provision") 3) Most people will be very wary of clicking links in a cold-call email. Both because it would take time, and because they're expecting to see a huge survey and/or a crappy app made by some schmo. To get around that negative bias, include an impressive screenshot of your app, and include all the questions of your survey, or if it is super long, include just the most interesting questions to pique their interest. Have those things be in-line, not as attachments. Doing these things will help make them curious and help convince them that it's worth clicking on the link (they'll trust you more). 4) If allowed in this project, look into other routes other than emails. For instance: A) Post it to relevant subreddits (https://www.reddit.com/r/AlphaAndBetaUsers, https://www.reddit.com/r/sideproject, etc.). B) Mail a small gift (e.g. a small chocolate) to companies, and have a note on the chocolate with an intro and a link (use an URL shortener) and QR code. Even if they don't go to the link on the note, you could do a follow up email in which you mention that you previously sent them the gift, which will increase your success.