Hubspot's got the best quick summary, in my opinion. They say buying an email is a bad idea because:
- Reputable email marketing services don't let you send emails to lists you've bought.
- Good email address lists aren't for sale.
- People on a purchased or rented list don't actually know you.
- You'll harm your email deliverability and IP reputation.
- You can come across as annoying.
- Your email service provider can penalize you.
Don't believe them? Ask Campaign Monitor.
Don't believe Campaign Monitor? Ask Constant Contact.
Don't believe Constant Contact? Ask Godaddy.
Don't believe Godaddy? Ask HostGator.
Don't believe HostGator? Ask SparkPost.
Don't believe SparkPost? Ask GetResponse.
Don't believe GetResponse? Ask Vertical Response.
Don't believe Vertical Response? Ask WhatCounts.
I could keep going...but you get the idea.