More than likely the seller didn't care about losing a pizza. What are they worth? a few £/$ or what ever currency you handle. But, think of it. 10,000 Bitcoins sounds a lot. Even if they weren't worth much back then. It still sounds like a lot. Well let's put it this way. 10,000 sounds like it could be more valuable than a pizza. Also, the pizza seller probably knew that Bitcoin was going to be popular among geeks. Which would make the value of it rise. So he probably knew he had a chance of cashing it out with a little profit. But, obviously Bitcoin has become much more than he hoped for. It is popular among non geeks and high up professions.
At the currency going rate of 10,000 Bitcoins. A pizza sold for $966,000. Is that a world record?
In that case he had to count that he would sell BitCoins or will exchange them for something. What gave to him such grounds? What facts? Prevalence among geeks? But why them to buy his Bitcoins, they can just mine it)