All fun and games! The digital world hasn't changed, but there are more ways to do things today, than yesterday - generally a good thing;
To the art world scenario; Cascading Trust is necessary to make this work; It is part of the world today and won't vanish until AI gets a lot better. To register an asset on any art chain, you need to verify its authenticity prior to registration. This means reliably ascertaining the identity of the owner (identity issue mentioned above) as well as reliably ascertaining the validity of the art (asset). In the latter case, you need physical inspection by an expert whose reputation is tied to the inspection results and who is also reliably identifiable (identity again!). As time goes by, and more inspections occur, they are attached to the asset on the chain. Sooner or later, there is a sufficient amount of validation to support the validity of the asset and the ownership. Nefarious inspectors are blacklisted quickly (if the identity component works). Fake assets that are validated and later proved fake damage the reputations of all verifying inspectors.
In terms of on chain verification, one could use high resolution photography to create an analysis of the actual tone gradients and stroke components of the art (color is too difficult, today) creating a representational map at the micrometer level which could then be hashed and stored as the fingerprint of the piece. A forger can fool the naked eye, but won't likely replicate the piece to this level of complexity (unless the replica was created using 3D printing technology). This is just meant as an example of a validation routine that could allow an end user to reasonably quickly determine if a piece was highly likely to be the same piece that is registered on the chain.
AI (much more advanced than today) could remove the need for human authorities, and increase the identifiers possible to fingerprint the work; but as with all tech, this AI technology could also help fraudsters to up their game too.
Blockchain transactions work best when there is verifiable trust in the IDENTITY protocols in use to identify the parties involved and the assets. This is no different than what happens today without blockchains; the improvement is that once identity is established, it doesn't need to be RE-established at every point in the supply/value chain.
Now; to answer the "What if I get scammed?" question: You are correct, you can't UNDO a transaction. You CAN update it. If you get scammed, then there was a failure of IDENTITY on the platform; There will necessarily be parties who have guaranteed one or more identities that were party to the transaction. Those parties are now legally on the hook for their failure. This could be a KYC provider, an inspector, or even the owner of the smart contract/blockchain company. You have recourse, and if the company isn't prepared (and insured) to handle this, then you probably don't want to use their services. Properly implemented; scammers will avoid these platforms due to their transparency and lack of anonymity.
In terms of the REFUND process; this works the same as it does today. No one returns to the seller and says "UNDO my transaction," they say "It doesn't work, I want a refund." It's a NEW transaction (just ask the sellers bank or ERP system); These scenarios are easily handled in blockchain.
As far as speed of transactions goes; sending funds door to door within a highly advanced banking system is AS fast as most crypto platforms designed for the same purpose (BTC was never meant to do that); the fees are lower and the most telling difference is that the speed and fees in crypto is the same whether you send it across the street or around the world. Try sending funds from South Africa to Mexico and see how long it takes, and how much it costs.