@Mike_Hakopyan - I’d be curious what others think too, but here are my thoughts:
- Agree with you that a list from a previous business, assuming it has no relationship to the current one, is probably not going to get a good response rate at best, and at worse Spam complaints harming the new businesses reputation before it even starts.
- It might not even be legal depending on the your local jurisdiction (unless the previous business was acquired or converted to newco).
In my opinion, the only viable way to “leverage” the list is to send an email from the previous business’ email service provider to encourage people to join the new site’s list. A few ideas to that come to mind:
- Have the founder formally “shut down” the old business, thanking them, and announce the new business and offer incentives to reward their past patronage if they subscribe.
- Do a promotional giveaway to announce the new business venture (again, from the old businesses’ email service provider). Collect emails as part of the giveaway.
- Create a simple one-page “landing page” on the old businesses’ home page directing them to the new business. People who return to the site might naturally migrate over. There’s a long-tail of email traffic from past campaigns that may still be in people’s inboxes.
Outside of email, the email address can also be used for retargeting ads to bring them to the new site if there’s enough affinity - but make sure to understand your ad platforms (fFacebook, Google) guidelines to be able to demonstrate that the new entity has access to the previous business email list (transfer of assets? etc).
Email works largely because of the “intent” and brand affinity behind it. I will take a smaller high quality list over a larger unqualified email ten times the size any given day. Any shortcuts simply undercuts its value - so what’s the point? If you want a million email addresses, there are plenty of shady places you can get that - but without the intent and brand affinity, it’s pretty much worthless.
Just be super careful - it’s a new business so you don’t want to start on the wrong foot and frankly, Klaviyo frowns upon it. You’ve been warned! 
Good luck!