As other comments have pointed out, not having the subscribers list is my non-negotiable terms if someone wants to take over. In fact, after I refunded everyone, I'll reset my server and remove all my DB backups so I won't even have a copy of it.
I know it doesn't make business sense for someone to buy a business without any customers. That's why I never explore the option of selling it.
That's an interesting choice. I'm sorry, I scanned the blog and didn't see this mentioned. It certainly creates a higher hurdle for a sale.
I'm sure you've considered all the options, but may I ask why you didn't choose to offer current subscribers an 'opt-in' option for being contacted by a new publisher? That way those who were interested in continuing with a new owner would be able to, and those who were only interested in your product wouldn't be bothered. (Again, I wasn't able to find the blog post where you mentioned being approached by possible owners, so if you answered this question there, I apologize.)
But if you open source everything (as mentioned in another one of your comments), that'd be great too.
I know it doesn't make business sense for someone to buy a business without any customers. That's why I never explore the option of selling it.