Card costs
Instead of or as well as a coin symbol, some of these cards have a card back where the cost is. It means that instead of or as well as $, your cards are involved in the cost. They can be anywhere, so long as you own them. Below the line, there will be a description of the cost.
You might think of Animal Fair having the option of an Action in hand as a cost. It's an option, card costs are not.
So to buy a Campsite, I can just fish two Victory cards out of my deck and/or discard pile, reveal them and discard them? Sure looks that way.
Ah, whoops that is unclear. The bit you bolded is just there to say in theory a card cost could include your cards in play ($0 cost Grand Market would be a card cost), or the number of cards in your deck. If one wanted to make such a buy condition it would be possible. It would be a bit weird, not much of an actual expense, but doable. I'll change the OP to say this.
So 'reveal and discard 2 Victories' is not specifying where to discard from because it's from hand, to follow the official way of writing instructions, and revealing them because of accountability.
Would 'discard 2 Victories, revealed' be clearer?