Well, that is a good question.
First, the image that appears on the website (NFTShowroom) it's a low resolution at most 600 x 800 pixels, all the original art is hight resolution (1K, 2K, 4K and up) basically for printing purposes.
If someone buys an original art, they will get the high resolution version, they can sell it of course (Since they have the big file) but since the art has been registered on the blockchain, a date and author has been stamped on. If they want to sell it later as their own, it must be in a different market, for example on Makersplace, if they claim to be the author, Makersplace checks all over the internet to verify that the art has not being published previously. If they find that the art has been created by other author, they simply ban the person from the platform.

One of the advantages of using NFTs is that anyone can buy art and re-sale it later on the market. So there is no point of copying and trying to re-sell as their own. It can happen, yes. But buyers can make a quick search on the internet to verify, also it is good to know what the author has been creating previously.

So know your artist first!