Microsoft allows you to install Windows without entering a product key, but that does not mean Microsoft grants you a free license to use Windows indefinitely. The installation may continue working with limited features, but Microsoft's licensing terms still require a valid license.
Regarding the product keys selling for 200 EGP versus Microsoft's price of over 7,000 EGP, I would be very cautious. These inexpensive keys may be unauthorized volume license keys, previously used keys, or keys obtained through questionable channels. Even if a key successfully activates Windows, that does not necessarily mean you have a legitimate license. Microsoft can also block improperly obtained keys later. Many volume license keys do lose activations if the original "organization" account is no longer maintained with Microsoft.
The only guaranteed way for a genuine license is by purchasing from Microsoft. Microsoft doesn't guarantee legitimacy and cannot be held responsible for any key purchased via a third party.