Thats a great question, in theory people could copy art and sell the copy, but most of the reputable platforms verify artists heavily before allowing them to tokenize (such as superrare, knownorigin, etc.) some of the more open ones such as rarible require the buyer to DYOR (do your own research) before purchasing as it is more decentralized but they have just included a verified system as well. There will probably always be a level of curation when it comes to art buying and selling because people want to support the original artist and take quite a lot of care when making purchases to make sure it's the real deal :) If there is any dispute as to who created the token first it can be easily verified on the blockchain or through other means if they have stolen a non-tokenized work.

At NFT Showroom we are verifying artists before they are allowed to tokenize.