You can install PHPAuction from the Fantastico web installer in cPanel. To accomplish this go to your cPanel and click on the Fantastico icon. You will see a list of available applications, click on PHPAuction and follow the onscreen instructions.

Since SiteGround no longer offers Fantastico, and PHPAuction is not included in Softaculous, if you are a SiteGround customer and you want PHPAuction installed on your account, you can request its installation from the Script Installation category of your SiteGround Help Desk.