How are the price signals of stocks in the stock market centralized? We can look up the price of a stock, like that of Google, and it gives a particular figure. I would like to know how the price signal is "centralized". What I mean is: if the trades happen between individuals holding the stocks, then how is the overall trading price arrived at? Surely, the individuals who are on the buy side and the sell side would have to "register" the sale with the SEC or something? That's the way it happens with other tangible items, like houses and cars, etc.

The stock price on the market acts as a global signal, which efficiently moves financial capital around. Without it, we would be left to our best guesses, which might take too long to converge on a single number, if it ever did at all.
Think of the stock certificate as a physical object being traded around. It may be traded on more than one market. The centralisation you speak of depends on the source for the quote. The quote, generally (some exceptions), is a last trade price (a delayed last trade price). It is not global. I have trading software that allows me to see quotes with less delays, I can choose to see it only on certain markets or last trade from any market. I see different signals than you do. 
The most direct answer to the question in the title is "they're not". There is no central, global signal. What you are likely referring to is just the (delayed) last trade price of the security on the markets that the quote service pulls from.