Inflation, exchange rates, taxes, purchasing power, personal income and circumstance pretty much all dictate that the true cost of a product will not be the same between two people even if they ostensibly paid the same “price”. The marginal utility of a dollar in savings is not the same for a person making minimum wage as a person with a 6 figure income.
It also creates incentive dilemmas for marketplace developers; Suppose someone offers a new competing product for a lower price, do you keep your price the same and lose future sales? With diminished future sales there is less to justify updating or improving your product for your existing customers.
That being said, none of this really has any bearing on how customers actually feel about their purchase. Certainly it is normal for people to feel like they got ripped off if they purchase something at full price only to have it go on sale shortly after.
Sellers will often try to hide this by price discrimination (where you pay a lot more to get a marginally better product, such as phones with different storage levels) or by simply running perpetual sales so that people always feel like they got a good deal, even if they might have gotten a slightly better deal if they had waited. Some retailers will even automatically credit you the difference if a product goes on sale shortly after you buy it.
It’s up to you how you want to handle pricing but I don’t think this is as simple as just saying “sales are unfair to people who paid full price”