Hello,
Thank you for your question regarding reselling a VPS or dedicated server. You could certainly 'resell' a VPS or dedicated server, but these would count as a new account under your existing account with us, if you have one already and wish to join them together. You may also wish to have any VPS or dedicated servers on their own AMP accounts for compartmentalization reasons.
Once you have the new account purchased, I would recommend looking into installing and using WHMCS and it's options. While we don't provide installation or support services for WHMCS we do have some basic information regarding what WHMCS is and how to use it. Please do keep in mind that this article was written with a Reseller account in mind, and some of it may not apply to the setup that you are describing.
Finally, I recommend reaching out to the WHMCS support team as well if you have more in-depth questions regarding ordering, installing, or configuring that software.
Gratefully,
Casey B.