[Idiot's Question] If a trader stares at his screen and sees his stock go from $100/share to $102, who decided the new price is $102, and how?

I understand that if more people want to buy (demand) something than sell it (supply), then it technically becomes more valuable.

However, if a trader owns stock worth $100/share and for some reason a demand for this stock breaks out and everyone wants it, is the new price dictated by buyer sending offers to the trader, or does the trader set the price he's willing to sell, or both?

In other words, if a trader stares at his screen and sees his stock go from $100/share to $102, who decided the new price is $102, and how?

Submitted June 08, 2016 at 08:18PM by harmonytarkovsky http://ift.tt/1XG2XWt