Well if you speak of price you probably mean something like the actual price at mtgox. Now if you wanted to change the displayed price at mtgox you would have to sell for let's say 50% of the usual price to your own account. But at mtgox or any other exchange you don't trade with a specific account. You always go through the orderbook. So to buy or sell to your own account at the manipulated price you would first have to complete all orders inbetween. At that point it does not realy matter if you buy your own stuff because you actualy did change the price for real.
the way a currency exchange works, the coins you would put up for sale would be INSTANTLY bought for MORE money than you asked for.
For example:
let's use Mt. Gox as an example, at this time of posting the prices are
Bid: $1048
Ask: $1050
Last: $1050
The
Bid price is the
HIGHEST amount someone is
willing to pay [for a bitcoin] at that moment.
The
Ask price is the
LOWEST amount someone is
willing to sell [a bitcoin for] at that moment.
The
Last price is the most recent transaction. (a 'match' between Bid and Ask)
Now let's suppose you'd sell a bitcoin for $500, this would count as an
Ask of $500. This will immediately be matched to one of the Bids that is considered the best deal for you. (and in case the best deal doesn't have enough money to pay you, it will partly proceed selling and than continue selling to the next highest bidder, until your transaction completed or until there are no more bidders in your price range)
Since the highest bidder offered $1048 and $1048 is more than $500, you'll get paid $1048 and the price will drop from $1050 to $1048.
Unless you'd sell millions worth of bitcoin, you'd not even make a dent in the price.
This is also where the market depth comes in.
Let's suppose there are 5 people (Alex, Brian, Cindy, Dennis, Elisa) that either want to buy or sell bitcoin.
Alex has 12 bitcoins, and he wants to sell 2 bitcoins for $1500 and 10 for $1600
Brian has no bitcoins and he wants to buy 0.5 bitcoins for $900 per bitcoin
Cindy has 2 bitcoins and she wants to buy 1 more for $950 and sell 1 for $1100
Dennis has 3 bitcoin and he wants to sell 1 for $1100
Elisa has no bitcoins and she wants to buy 5 for $1000 each
The market debt would look like:
Buying Volume| | |Buying Price| | |Selling Volume| | |Selling Price |
5 | $1000 | 2 | $1100 |
1 | $950 | 2 | $1500 |
0.5 | $900 | 10 | $1600 |