Something trivial as matching people based on interest. We have social networks and various marketplaces but we are still unable to pair/match people based on desired activities, items for sale, services, relationships(online dating), even jobs.
Imagine you have some ancient toaster you are about to throw in the bin because it is old and you have no use for it and it has no value on the market. Yet, on the other side of the planet, there is a guy who is desperately looking for exactly this toaster because of #reasons. Yet, these people will never be able to find each other to trade.
Yes, there is ebay and whatnot, just like there is tinder for dating, facebook for socialising, various platforms for job hunting, but all these platforms are extremely inefficient in actually delivering on the promise of matching people based on the supply and demand.
The search engines all these platforms use are all very primitive and completely unable to provide the desired service. They are essentially all the same, they just cater to different markets. But there is little technical distinction among them.
The toaster example is a completely trivial one. You can easily expand it to a job where you need a person with specific skills and experience. But you will simply never be able to find that person via any of the existing pathways. Except sheer luck and word of mouth.
This can be likely solved via something like brain implants where we can be connected to the internet and immediately provide necessary context or answer some questions to build a better profile as a "supplier" or "buyer" that could allow a better match. But we're infinitely far away from it.
And this is just one of millions of such small problems that are really hard to solve.
The advertising companies all use tracking to try and mitigate this as much as possible so they can offer you the most likely product or a service that you actually might be interested in buying. But again, these are very primitive solutions.