This is useful up to a point. One constraint is whether the goods are fungible. (Rental location is not.) Another is whether price differences even exist.
> What is the problem with paying for what you are willing to pay?
"Free" markets are a fiction. Transactions do not exist in a legal vacuum. (Except perhaps for cryptocurrencies, which exemplify why such laws exist.)
The fact is that some markets are dysfunctional. Cartels, monopolies, and monopsonies are generally seen as evidence of market failure.
There are sound reasons for regulators to limit price gouging. Just because retailers have pricing power does not mean they should be allowed to exploit it to the maximum degree possible. For example: company scrip has been outlawed for many years, even in the USA. Likewise, antitrust laws restrict -- or are intended to restrict -- the ability of retailers to collude in fixing prices at whatever the market will bear.
Consider a real-life example. Imagine you bought a diamond ring for your fiancee. You paid $10,000 for a tiny stone because a cartel has been restricting the supply for 100 years. Six months later, chemists make diamonds in a lab and break the monopoly. You can now buy the same stone for $500. Do you feel ripped off? Or do you feel that the $10,000 you paid represented market value, and put the humiliating loss down to poor timing?
("Same stone" is not quite accurate, because for the time being de Beers is still trying to market "natural" stones at the original price. They don't have much choice because their stockpile represents an enormous unrealized loss. Behind the scenes, you can bet that they are trying to get rid of them any way they can. In crypto parlance, they are left holding the bag.)
Now substitute "life saving drug" for diamonds, and "pharma bro" for de Beers. (And recall that the price gouging was legal, Shkreli was jailed for an unrelated fraud.)