I think the answer your looking for is FAITH.
The first person to sell PIZZA for Bitcoin had FAITH that he could use that bitcoin to purchase something as well... if not immediately then in the very near future..
Faith and TRUST are the back bone of MOST currencies, not just BITCOIN.
I absolutely agree with you.
But if in a case with dollar or ruble, our belief is supported with state forces. Because of what there was a belief of the seller, what he thought he would buy something for bitcoins?
This is the thing about what we need to think

For example the belief in God : many people has it from stories of relatives or reading the bible.
And what was a reason of trust that the first who started selling something for bitcoins?
P.s. again I'm sorry for my English
