ASSUMING you obtained the list in a manner that lets you legally email the people on it, I'd imagine the general guidelines are the same as with a warmer non-purchased list. I'm hoping you are saying "spam" in the casual sense of "email blast".
1. Present a genuinely good deal. Consumers aren't dumb and you lose their trust if you tell them something is a good deal and it they know it isn't...especially if you nosed your way into their inbox to do it.
2. Don't spam.
3. All marketing like that is a numbers game. I've never purchased a list, but based on my experience with lists I've generated myself (customer signups) that I've later emailed, open rates are anywhere between 3 and 30 percent based on how click-baity the subject is. Click throughs are 5-10 percent of those. Orders are maybe 5% of that. So if you start with a list of 100,000 customers and you get optimistic responses, you're talking 150 signups out of 100,000 emails. That's optimistic. It sounds like you're talking a lot colder than your own customer base.
4. Don't spam.
5. Don't spam.
But seriously, don't spam. You can send legitimate bulk email. You can get your ad included in other people's bulk email (I just had one included in a blast to the forum members of another forum.) These are going to be much better ways of generating conversions that actual spam.