Purchasing a legitimate retail key is legal. However, Microsoft’s terms of service generally prohibit the sale of OEM (Original Equipment Manufacturer) keys as standalone products.
Beyond the legalities, there is a massive security concern. Windows 7 no longer receives security patches. Using a "newly activated" version of an obsolete operating system is akin to locking the front door of a house that has no walls. Without modern defenses against ransomware and phishing, a Windows 7 machine connected to the internet is a liability. window 7 activation key walmart