i purchased a win8.1 pro key from g2a.com. how do i know if it is a OEM or retail key?
It's easy to check once it's installed:
Right-click My Computer and select Properties. Scroll down to the bottom of the screen and find the Product ID.
The Product ID should be in the form xxxxx-xxx-xxxxxxx-xxxxx. What are the 3 characters in the second group (if those 3 characters are OEM, then you have an OEM version).
OEM - OEM SLP or OEM System Builder (MGAdiag will distinguish in the License Type field)
007 - VLK MAK
033 - VLK MAK
049 - VLK MAK
050 - Technet Plus Enterprise MAK (do same key used to activate Professional give this CID?)
065 - Technet/MSDN/Bizspark Windows 7 Ultimate
066 - Technet/MSDN/Bizspark Windows 7 Ultimate
067 - Technet/MSDN/Bizspark Windows 7 Ultimate
074 - Retail Windows 7 Ultimate
112 - Retail Windows 7 Home Premium (conflicting info says MSDN)
177 - Retail Windows 7 Professional
220 - Technet/MSDN/Bizspark Windows 7 Professional
230 - Technet/MSDN/Bizspark Windows 7 Home Premium
231 - Technet/MSDN/Bizspark Windows 7 Home Premium
292 - Retail Windows 7 Ultimate?
293 - Retail Windows 7 Ultimate? (maybe Academic)
339 - Retail Windows 7 Home Basic
838 - Technet/MSDN/Bizspark Windows 7 Professional
896 - Retail Windows 7 Home Basic N
929 - Retail Windows 7 Home Basic E
956 - Retail Windows 7 Ultimate (Genuine Advantage store?)
Also, just confirm with the info you got from g2a.com. The invoice should state if it's OEM or retail.