Of course. But it is still an arbitrary mechanic. Where does the merchant stores the gold pool gold? Does he pull it out of his arse? Even if he did that, it should still be available for looting after he is killed.
And surely robbing a rich merchant should be more profitable than robbing a poor one. The problem of getting too much gold should have been fixed in the stealing mechanic instead. We should consider an option to disable the separate gold pool after 1.0.
Anyway, enough game mechanics rambling. Back to topic.
I guess Jyby meant "buying from vendor" when he wrote "Sell something to vendor". Otherwise the statement indeed doesn't make much sense.
Just want to point out that the second bolded statement doesn't follow with any kind of necessity from the first. You seem to be taking a bunch of matters of design philosophy as axiomatic that I think warrant being discussed explicitly, since there doesn't appear to be a separate thread for them.
Even setting aside the philosophical question of where the optimal balance point between ease of design, realism/plausibility, and being fun in practice lies, there's the fact that the whole point of RPG systems--the stats and skills part--is to provide a model of the real-world mechanics of being a person and doing stuff. Some aspects are deliberately abstracted away to make the model less cumbersome, easier to test, easier for the player to understand, etc.
Specifically, I think Bethesda designed the system as it is to represent the fact that you will never actually be able to walk into a merchant's shop and walk out with their entire life savings. In fact, I'd argue that robbing a very successful merchant could very well get you less
actual cash because savvy businessmen have better things to do with their money than store it under their mattress, and tend to keep meticulous track of how much they've allowed to accumulate in the store.