You just invent a thing called notaries. You'll have to go back in time to do it since they already exist but that's besides the point. Anyways, a notary puts their reputation/legal liability on the line and counter signs your transaction. Since they've been granted license by a real legal entity with guns and jails they commit to verifying a transaction occurred at X time.
Monies for the transaction can be put into escrow with a third party (you'll need to go back in time to invent that too). When the notary notifies the escrow holder of the signatures they can release the monies.
When you have a legal system to enforce contracts and people put their own money or legal liability on the line two parties don't need to trust one another to conduct transactions. People buy expensive shit all the time. They don't need to trust each other with the money or valuables because they involve third parties that proffer up (staking if you will) liability to let the two parties conduct transactions.
The blockchain model replaces the staked liability with "expensive" proof of whatever. The only problem is monied entities can these proof systems. They can afford to run a majority of miners, stake the most (or spam stake offerings), or whatever otherwise supposedly expensive resource. They're only expensive resources if you naively assume all mining will be done by randos in their bedrooms.
Blockchains are just a way to do something almost no one needs in a slow expensive way.