Imagine you are buying books at Amazon and it says.

Total to pay: $50USD or 50BTC? If 1BTC > 1USD you will choose to pay $50, correct?

Variant B:

Total to pay: $50USD or 25BTC? 

It means that 1 BTC = 2 USD but how can it be? There must be something

1) that you can buy only with BTC
2) is scarce
3) is in big demand by millions ( to provide smooth exchange rate)

Point me where I'm wrong...

PS. Bitcoin idea is simply amazing!