When buying a business, what is the best practice for using the email list that came with the business? Can I just upload the contacts into my CRM and begin emailing them?

First, we want to be clear about this: do NOT buy an email list and then start emailing them. In some countries this is illegal.  And in general, it's just a very unethical business move.  People sign up to a list for a specific reason. If you didn't give them that reason personally, then you're violating their trust on a fundamental level.  Also, purchased lists very rarely convert very well.  It's just a disastrous decisions to make. 

Okay, with that out of the way, here's what we have to say if you have purchased an entire business that included an email or customer list:

Is this just a change in ownership with things staying the same to the consumer, or is it an acquisition where things fundamentally change for the business? If nothing is changing, then you should be good to go as the customers have already "opted-in" to the original list.Change of ownership wouldn't affect that.

You could also send an email notifying people of the change in ownership, why they are receiving the email and ask them to confirm to be on the list. That way you have express permission as a back up.  Also, it's just good practice to introduce yourself. 

If it is changing ownership, but the corporate name is the same you should be fine. Issues come up when it changes hands and the company name isn't the same.