It's not a matter of law, it's a matter of contract and negotiation with the seller and the buyer, and the rules imposed by the owner of the voluntary market place.
Many vendors in the world allow and openly publicise this practice - it is good for the vendor, because it means that the prospective buyer does not delay a purchase hoping for a sale soon, but can buy now and get the sale price later if there is one (and if they also remember).
It means the vendor might even sell-out and so not need to have the sale.
Also, if you can buy a game and get a refund if it turns out to be rubbish/boring/easy then you are more likely to take a risk on it - you only have your time to lose.
This strategy makes it easier to sell, and vendors who are confident in the quality and value of their goods can get much benefit as sales are easier.