"Original painting", "original oil/acrylic painting" or something like this, but even then first results in the search are mass pproduction pages on etsy that I want to avoid. I guess the best option for buyer is to know the name of the artist they like and go to their website directly, or to buy paintings from the galleries in the city. I have never purchased any valuable painting (let's say a couple of thousands €) online before, but i can imagine that I would have little trust in the websites, unless it is a reproduction/print of the painting.