As far as domestic law is concerned, this is not completely illegal, but game developers (GGG) prohibit it. If you are a regular player and you only have RMT and other players with less POE Currency , you can basically do it.
The advice is just to do this, just like you are really dealing with selling your players, for example pretending that you want to sell him a rare/unique item. Just tell him and ask if he still wants POE Items , no one will doubt you.
If you buy from a big store, but the risk increases, you may find yourself on the GGG radar. I prefer to use, which is a player-to-player market where you can choose which trading objects to trade. I also like the layout of this website - I feel very comfortable buying there.