I am looking for good Windows 10 version to buy, but there are different versions of this OS.
For example what is the difference between MAK an OEM versions?
A Multiple Activation Key (MAK) enables you to activate multiple installations of a product with the same key. MAKs are generally used with Volume Licensing versions of products.
OEM Key These are Original Equipment Manufacturer keys that allow multiple activations.
OEM is usually attached to a PC (COA stickers) and these are selling rarely but these are expensive. If anyone is selling OEM on forums, probably it's stolen or hacked.