I guess if it is overbidding buys by a minute amount, and underbidding asks by a large amount, then the price would be lowered. Basically instead of the price moving between X and Z, the price now moves between X and Y (Y is between X and Z). Everybody sees the highs lowered, and loses confidence in speculating on bitcoins, and so the price drops.
Hmm, I disagree. The sole fact that he is overbidding current bids by a small amount means that what I can get for one Bitcoin increases (by a small amount). The value of a Bitcoin is what anyone can sell them for on the market, not what some participant on the market is buying them for. The price of currency
A can be brought down only by selling
A, not by buying
A.
If X is the is the current bid, and Z is the current ask, and someone is offering (asking) to buy Bitcoins at Y (which is between X and Z), the spread changes from Z-X to Z-Y. No change in the price of Bitcoins happens, only a change in the spread. According to bitcoincharts.com, the current spread at Mt. Gox is .66%. Mt. Gox charges a 0.6% fee, which means what there is potentially 0.06% to be made by trading Bitcoins. If the spread decreases to below 0.6%, someone is making less money trading Bitcoins than they are paying to Mt. Gox in fees.
If I recall correctly, the fee drops to 0.55% at some trading volume, but you get the picture.
I think you are just looking at one moment, I was talking more of over a period of time.
Say on monday there is no bot, the price bounces from 5.0 (X) to 7.0 (Z). Average price is about 6.0
Then on tuesday the bot appears, price bounces from 5.1 (X+a bit) to 6.0 (Y, Z-a bunch). Average price is now about 5.55, so the price went down.
I believe you need to separate the asks and the bids in your examples. You can't mix up the asks and bids and say the value is between the asks and bids. The value of Bitcoins is not somewhere between the highest bid and the lowest ask; it's the highest bid. In your example, X is the highest bid and Z is the lowest ask. The highest bid (X) is what I can sell Bitcoins for at the moment (to the one bidding), ie. the value of Bitcoins. The lowest ask (Z) is what someone else is willing to sell his Bitcoins for.
The value of Bitcoins (or any currency) is determined by what it can be sold for, ie. the
bid price. Not what other people are offering their Bitcoins for, the
ask price.
On Wednesday, Mr. Speculator, who was planning to buy some bitcoins, sees the price dropped, and so he does not buy bitcoins (hoping they will drop a bit further). Mr Miner, who needs to pay his bills, sells anyway, so the price drops again.
Mr. Speculator does not see that the value of Bitcoins has dropped. He sees that what he can buy Bitcoins for is now less than it was before, but what he can sell Bitcoins for is slightly more than it was before, ie. the spread has decreased.
Of course, psychology can have any effect on the market. If Mr. Speculator doesn't realize that only the spread has decreased, he could conclude what you do and sell all his Bitcoins, thus causing Bitcoins to decline in value. But this doesn't mean the decreased spread has caused a decline in the value of Bitcoins, it means that Mr. Speculator's ideas about finance has caused him to sell his Bitcoins, which then causes a decline in the value of Bitcoins.