That's why it should be not 'just' expensive. Either it should not be available or only under certain conditions.
It isn't available to you if you don't have the gold to buy it.
What's the difference between a book you don't have the gold to buy, a book the vendor doesn't sell and a book you cannot use because of your level?
They are all unavailable to you, the only difference is that one of these situations is healthy for the economy because there are things being sold you can only buy if you spend a lot of time and efforts into acquiring it.
If you are able to buy everything a vendor can sell, there is no gold management to be done. If you are able to buy items you cannot even use, there is simply no economy at that point.
