Serafína,
I totally understand your confusion because I lived abroad for 11 years and the idea of handing a bag full of dollars to the seller would be downright rejected and totally suspicious.
Here, in general, works differently. The typical scenario is as follows:
You withdraw the necessary amount from your bank account for each step of the operation.
The only thing to consider, in my experience, is that except for small amounts, when withdrawing you need to let know the bank branch in advance, so they have enough usd to pay you.
Sometimes the bank has meeting rooms where you can do the operations, but you need to get the seller and the escribano to agree to do it there. Usually, you do it at the escribano's.
It's common here for people to store this kind of money in security boxes though, so to get the seller onboard you could open an account in their bank and branch, so they don't have to get out of the branch with the cash.
This is not to say that you can't do it electronically, you just need to get the other parties to agree to do it. There is no rule against it, but I guess culture and history still make it less appealing than cash.