That of course assumes there's a price that is "fair" and you have a magic ability - personally or "as a society" - to figure it out while ignoring the markets (because the markets are "unfair"). I wonder how one does that and doesn't end somewhere in the vicinity of Detroit.
As for my 'personal' judgement on what is fair, I always defer to those entering the terms of the contract, so long as those contracts abide by law. If you offer me $10 an hour to do something I might ordinarily charge $100 for, and I accept that $10 an hour, and the $10 an hour does not break any law, then to me, that is fair. I am by no means authoritative on the subject though, so I appreciate any insight you may have.
I'm not saying the markets are unfair. I'm saying that's what exploitation theory assumes (which theory I am in no way supporting or endorsing) - the base for "exploitation" theory is that the market price diverges from the supposedly "fair" price, and the difference is the measure of "exploitation". This presupposes that a) "fair" price is known and b) it is different from market price.
1) The general (non-Marxist) idea is not that 'the market price is unfair, so we should set different prices ourselves', but rather that the market isn't magic, and does not appear ex nihilo, but is impacted by many things. Some of those things (such as law about hiring, firing, welfare, safety, and so on) can materially affect the negotiation in a way that is patently not fair. For example, if unions had the kinds of powers some ascribe to them (ability to set wages however high they wanted with all staff being unfirable no matter what they did, with employers being powerless slaves) that would be obviously unfair. The power dynamic in many countries (including the US) is roughly that slanted at the moment, but in favour of employers rather than employees - in the US, for example, this is the effect of things like no cause firing, binding arbitration agreements, 'temp' positions and 'internships' being allowed to be offered in place of actual jobs, no real socialised healthcare, limited unemployment insurance in most states, low minimum wages, no 'union shops' in most states, anti union laws, limited occupational health and safetly laws in many states, and so on. So, to make things 'fair', these things have to be fixed - then the market will be 'fair'.
2) The general (Marxist) idea is not that 'the market price is unfair, so we should set different prices ourselves', but rather that capitalist-worker relationships are always unfair and exploitative due to the worker always having to sell their labour for wages, rather than capital, where the capitalist simply gains more money through the actions of capital - neatly setting up a coercive power structure. The (tl;dr version of the) Marxist response is not that this means that we have to set 'better' or 'fairer' levels for wages, but that we have to destroy capitalism and abolish wage slavery.
>>> can materially affect the negotiation in a way that is patently not fair.
Here you have to define what "fair" means. One of the definitions would be using extra-market coercion to influence the price - see unions example - is not fair, since it uses forceful coercion to benefit one side. However, unions often claim they need the coercion to reach "fair" prices since otherwise it is "unfair". So what is "fair" here? How you find out if voluntary agreement of two people is "fair" or not?
>>>> we have to destroy capitalism and abolish wage slavery.
Since the only other alternative that we've seen so far is non-wage slavery (at least until we find enough people that agree to work for free that we can create non-scarcity economy) this makes this definition of "fair" rather unappealing. The problem here is that voluntary structures tend to become markets, and calling non-voluntary arrangements "fair" has to rely on notion of fairness of those who apply coercion to support the non-voluntary nature, which very soon devolves into very peculiar understanding of "fairness".
