Most likely you don't buy IP addresses outright. You sign a business Internet contract with your ISP that has a fixed IP address. You essentially lease that IP

Most likely you don't buy IP addresses outright. You sign a business Internet contract with your ISP that has a fixed IP address. You essentially lease that IP address from your ISP. You will stipulate in the contract that the IP address will be used to send/receive emails, because otherwise ISPs may block SMTP traffic completely. Once you get big enough, you can consider buying an IP address prefix, getting your Autonomous System number, setting up peering, etc. For example Mailchimp seems to have AS14782.