The cheap license landscape can be confusing. Here is a practical breakdown:
Most $5-15 keys are Volume License or MSDN keys resold individually, which violates Microsoft ToS. OEM keys from decommissioned PCs are technically single-use and tied to original hardware. Retail keys are the only fully transferable type, but they rarely sell for under $30.
How to check what you got: Run slmgr /dli in elevated CMD to see license type (Retail, OEM, or Volume). Run slmgr /xpr to check expiration status.
Risk assessment: The key might work initially but get blocked later. You will not get banned or bricked though - worst case Windows goes back to unactivated with a watermark. For peace of mind, buying from Microsoft Store or authorized retailers is the safest option.