Right, the thing is if I want to involve a third party (and I definitely want that), I wouldn't use a decentralised system to begin with, because decentralised means inherently inefficient and therefore expensive.

Especially since you have to trust a third person anyways and once you're trusting anyone, you lose all the blockchain guarantees.

False. The third party can't take the money, it can only be given to the receiver or back to the sender. That's how multisig works.

Correct, you have to trust the extra signer to make a good faith representation of what happened in reality. That makes the transaction inherently trustful.

Sure its inefficient. For example the complete Tezos network uses around 10kW of power while a centralized solution might only 1kW. Relevance: None.

Electricity is far from the only measure of efficiency in financial transactions. Cost overhead is another one.

Sellers do not want, but buyers have the money and hence the power so sellers mostly just have to deal with it.