You are playing the buy phase incorrectly.
In the buy phase, you always play all of your treasure cards, one at a time, before you buy any card. In fact, you are not allowed to play any more treasure cards after you buy a card.
You never play specific treasure cards to buy specific cards. Don't think of treasures as the currency you use to buy cards. Rather, treasure cards are just cards like action cards are, except that you play them in your buy phase rather than in your action phase. When you play a Silver, it produces $2, just like how playing a Festival produces $2. After you have played all of your treasures, you will have some total amount of money available, produced by all the actions and treasures that you have played. Then you proceed to spend that money on cards you want to buy.
So when you play Venture, you first get $1 added to your pool of money, and then you reveal cards until you reveal another treasure card and play that as well.
If you play 3 Idols, you play them one at a time and fully deal with each Idol when you play it. So the first one will give you a boon, the second one will give your opponents a curse, and the third one will give you another boon. Only after you are done playing all of your treasures and dealing with all of their effects do you buy cards. The treasure cards and the instructions on them never care if you actually buy anything. You can play a bunch of Idols just for the extra benefit and then never buy a card.
From the base-game rulebook (second edition):
First you can play any number of Treasure cards from your hand, in any order. Treasure cards say "Treasure" on the bottom and have a yellow banner. You play one by moving it to the "in play"
area; you probably will not announce your Treasures, though you can if you want. The Treasures have no text, just a big coin with a number on it. You get that many coins to spend this turn - one coin for a Copper, two for a Silver, three for a Gold, indicated as $1, $2, $3. The amount is also in the corners at the top of Treasures. You do not have to play every Treasure in your hand (but only get $ this turn for the Treasures you play).
Then, you can buy one card, costing as much as you have or less.
You cannot go back and play more Treasures after buying a card; first play Treasures, then buy.
The treasure cards in Prosperity follow the same rules as the treasure cards in the base game; it's just that in the base game; no treasure cards have effects other than producing some amount of money.
