You're free to sell computers installed with Linux Mint, as long as you installed from the official Linux Mint ISOs and aside from possibly having installed any needed additional drivers have not customized the installation. If you want to sell computers with a customized installation of Linux Mint, please contact the Linux Mint project lead, Clement Lefebvre, at email@example.com
beforehand to get permission to use the Linux Mint trademark, artwork, and other branding and copyrighted material in your customized distribution.
Probably best to install Linux Mint 13, the current long-term support release, till Linux Mint 17 arrives later this year (it is the next long-term support release). You can download it from here: http://www.linuxmint.com/oldreleases.php
As you are in the USA, please use the no-codecs version for installing to avoid you getting into legal trouble for selling computers with software that possibly infringes on patents registered in the USA.