Thank you all for the answers. It confirm what I thought. There is still a need for 3rd party for "real" (physical) transactions.

Are you sure ?
I have thought about it and I have come to the conclusion that yes, we must have a third party.
Maybe I'm wrong, here is my reasoning.

Let's say I sell to you a highly expansive painting. We will set an appointment. The day comes. I'm putting the painting in my car.
I met you. But I don't know you. Maybe you have a gun ? If you have a gun and I don't, it will be very easy for you to steel my painting without paying.  (there is no police here because police is a 3rd party)
So, I will grab a gun AND a 2nd man with a gun also, to be sure that my painting is safe.
But you will make the same reasoning on your side. So you will come with your gun plus a friend and a machine gun, or a riffle.
And I will make the same reasoning, and so, I will have to come with at least the same destruction capacity than the value of the painting, so that it neutralize any wish to steel it.
So we will both come with huge fire power and men to help us. Those men can be considered 3rd party.
That's a kind of byzantine general problem.

But, in fact, it cost too much to deal transactions that way. So we will both give our guns to a 3rd party that will be set as judge in case of conflict. It's a 3rd party, and economically it's the only (is it ?) solution ...

Anyway, no matter how I try, I alway end up with a 3rd party for real transactions. There is no way any algorithm can help us. Unless we can built an automated justice with a robocop.

Do you agree ?

Hello,
bitcoin remove third party in transactions. Great. But does this solve the trust problem ?

Let me explain. Since I started using bitcoin a few years ago, I have the same question raising again.

Imagine I want to buy something expansive (a painting for example) and physical (not a de-materialized good) to a distant seller and pay with bitcoin. Imagine we live in an anarchy, there is no third party at all (no law). How can we make this work ?

If I pay and the transaction is written on the blockchain : the seller already have my money. Put aside is reputation, if he don't send me the package, my money is lost. Unless I have checked the real physical address of the sender before (but if he lives in a distant place this is difficult). So there is it, with any third party law enforcement, my money is lost.
If we reverse the situation and that the seller send the package and I don't want to pay ... all the seller have is a postal address. Even more difficult to get is merchandise back.

For a virtual good (a book, a video, etc...) we can have smart contracts that enforce a fair transaction. But for real physical tangible goods ?

So here I only see one solution : a third party that will handle conflicts.
Traditionally it is the bank.
We usually say that bitcoin remove the third party. Is it really ?

Is there any system that can do that ?
Sure, we can add a decentralized reputation system that will minimize conflicts, but it cannot avoid them fully.
The other solution is to build trust, step by step. First I buy a little thing, then a bigger, then a bigger, till I reach the real size of my expensive good I want to buy.
Should we add a kind of justice/police to the bitcoin to make it really work in real world ?