It's always allowed to buy anything from anyone. However, you might not get what you expect to get. The shop might cheat on you. Kinguin seems to be a platform (Hong Kong based) and the seller might be someone else who just tries to cheat on you. The keys you buy might not be redeemable. You might lose the $2 and not gain anything. This is happening quite often.

If you are not sure and if you rather want to play it safe, don't buy there. If you feel adventurous, try it out, but don't complain later if something goes wrong.

You could ask the publisher of Mount & Blade: Warbrand if they deem it possible that someone sells legitimate keys on Kinguin for this game. This would not give you 100% security though.

Umm.. King + Penguin?