It tells us that there isn't a big enough market for regular stores to sell Black Widow shirts. It's all about supply and demand, babe. There's a reason they used to sell natural snack crackers at my local grocery store but stopped selling them - I was like, the only person in town buying them. It sucks to have to go out of my way to get them, but hey, that's the economy for you.
A quick lesson for you, babe. Supply and demand is something obviously too complex for you to understand. Too often it’s used as a racist or misogynistic argument (why there’s more white/straight tv characters, why there’s merchandise for white Disney princesses and not POCs, why there’s no Black Widow movie.) The law of supply and demand is coming into play, but not in the way you think. Demand does exist.
The fact is that people want Black Widow merchandise. The demand exists. It’s not a case of it existing, it’s a case of being blind and deaf to the demand. If there was no demand then there’s be no reason for the dozens of online shops to come up with a design. You can’t sell what you don’t make, and the fact that Marvel isn’t marketing Black Widow is the reason their not selling it, not visa verse.
This is an example of demand. It’s a lost opportunity for Marvel because they don’t have a supply to fill the gap. On their end of things it’s stupid because it’s a money making opportunity they’re missing out on. More importantly it’s a message that women, both the characters and the audience, don’t matter to them. It’s ignorant to think that women aren’t interested in superheroes, that they don’t have the desire and ability to spend money on merchandise. Look at events like Comicon, for example. 130,000 people, and more than 40% of those are women.
There is a demand that’s not being filled. That’s a failure of understanding the supply and demand model. In your cracker example the demand was less than the supply. In this case the supply is nonexistent and therefore other sources are taking up the slack.