As far as I am aware, Microsoft do not offer a specific license for Windows on refurbished PC's. This is just a term used by sellers of OEM to try and represent the software as a direct alternative to retail, which it is not. This is due to restrictions on its transferability, hence why OEM licenses are significantly cheaper than retail.
As long as you are happy with the restrictions an OEM license places on you and that the license key has not been used, then yes, you can use it for a self build PC.
Please note that once installed an OEM license is effectively bound to major components of the PC. If for example, your motherboard was to require replacement, you would be unable to activate your license upon reinstalling without calling Microsoft to explain the situation. It is then at their discretion whether to grant activation or not.
If this has been purchased in an "online auction", the only way to verify that the key has not previously been used is to contact Microsoft to verify this along with the license legitimacy. I would suggest you do this prior to opening the package.