...or to be more precise, the aggregate activity of all buyers and sellers in a market.  If there's more than one buyer and/or more than one seller in a market (the usual case), then it seems that your model for determining fair prices breaks down. 

Embrace the yin and yang of supply & demand, man!  There's nothing fairer than a well-operating market for determining 'fair' prices.  No government 'levelling of the playing field' necessary.

So long as their pricelines match somewhere, they will come to an agreement.  I'll switch terminology and say that both are equally "worse off" if the area of overlap on the priceline is bisected; at this bisection is the objective "worse off" point.  This would be the point at which neither person is, comparatively speaking, better/worse off than the other.

If I've got this right, you're saying that if the seller is willing to sell for $50 and the buyer is willing to buy for $100, then they should reach a deal and the fair price is $75. Right?

My first problem with this model is that you said that we are assuming perfect information, but you didn't seem to consider how this impacts the prices demanded by participants in a market. In a functioning market, the seller should be willing to sell for no less than whatever price he thinks "the next buyer" is willing to buy for. Likewise, the buyer should be willing to buy for no more than what he thinks "the next seller" is willing to sell for. Sorta like a Vickrey auction .

In this case, with perfect information and a fluid market, both "the next buyer" and "the next seller" are basically the market price. So if the buyer and seller make a deal, there should be no ambiguity in the price and no way for one or the other of them to get the better deal.

I think you have interesting situations if the actors have imperfect information, or the market doesn't work well (e.g. small community or monopsony).

Some of your description inspired the following model of an actor with poor information:

• The actor does not have a clear idea of what the market price is.
• The cost of postponing the transaction is high (ideas of objective/non-market value might come into play here), so the actor is willing to error on the side of getting a worse deal just to make sure he gets some deal.

As you suggested, many transactions are asymmetrical. Consider a situation with a "big guy" (BG) who engages in high-volume market transactions and a "small guy" (SG) who engages in rare/small market transactions. For instance, BG may be a large employer and SG is one of his employees. Conversely, BG may be a retail outlet, while SG is one of their customers.

BG will typically have more information about the market, both because he has gathered this information in each of his transactions and because it is worthwhile for him to do market research. Conversely, SG may be pretty clueless. In this case, I'd expect SG to typically get the raw deal (i.e. the price will deviate from the perfect market price in a way that favors BG).

Furthermore, if SG is both uninformed and impoverished, he may take whatever deal BG offers simply so that he can quickly get some item that he urgently needs.

This model has plenty of implications, and I can only scratch the surface here:

• financial success may result from a person's ability to manipulate this dynamic (as opposed to being ameasure fo their contribution to society). It is good to have better information, better bargaining position, and to manipulate the other person's uncertainty (e.g. by spreading misinformation and appealing to emotion, among other underhanded but common marketing strategies)
• equality (and market efficiency) may be promoted by providing "the small guy" with better market information, or by addressing his anxiety about not failing to complete his transaction immediately.

Even if you consider the above issues to be in need of action, there are myriad strategies to address them. I'm not convinced that state regulation would be particularly effective. I am especially unconvinced that political activism (campaigning and lobbying) would be more effective than attempts to directly address these problems.