what I don't understand is how it all started. noone would buy something that has no value (the first mined coins). so who bought the first coins? It might have started as a pump and dump, and once this caused a valorization it caused a chain reaction.
Noone designed it as a pump and dump scheme. It was designed as a digital currency (as were all the digital currencies from the early 90's).
It was obvious that an exchange was needed to determine a price for it, but even before one became available, the first transaction occurred when a miner offered 10000BTC for a couple of pizzas. No pizza shop accepted them, so an aspiring bitcoin user paid fiat for the pizzas and made the trade to obtain his bitcoins. That determined the first defined price (about .2 cents). Interestingly, it wasn't zero before that really...it was undefined.
so one guy bought BTCs because of something like a joke, but how did the market build itself? who started to buy coins, and why?
Hmm. It was fun, but I wouldn't call it a joke. The digital currency work all thru the 90's was very serious stuff.
I guess the answer to why it built itself was that people realized it had a future and wanted to be involved to learn about it. I bought into various digital currencies back as far as the early 90's just to be involved for the same reasons. Then exchanges came along...
intresting, but I still don't understand how this could cause the initial "pump" (while I can perfectly understand all the following, once there was the first one)
You have found a fundamental value problem, which has been discussed here at length earlier. The fact that bitcoin has value, contradicts Mises' regression theorem, which basically states that the value must be based in part on yesterdays value, and ultimately, from the time deep in history, when some commodity got exchange value because it was the preferred commodity to hold for later. (Discounting debt as money, which could have existed before or after or at the same time).
Some believe that this means that bitcoin must have some value for direct use, in addition to it's money (exchange) value. I do not agree, I think the vision of the future usage of bitcoin as money, was enough to give it initial value, and that the regression theorem has to be shelved.