Actually I have witnessed just that on many occasions. Technically both parties must be "willing", but as GomJabbar mentioned, often one of the parties essentially "agrees" against their will or against their better judgement.
If an employer needs an employee more than the employee needs the employer, it is not at all unusual for the employee to essentially dictate the terms of their employment. (not particularly common in a capitalist society, as workers tend to be in a weaker position than employers, but it *does* happen)
Once again, this is only true in theory. In practice, an employee often finds themselves accepting positions they really don't want (ie because they just need a job of some kind), and employers sometimes employ people that they don't really prefer because they just need some work done and can't find someone more to their liking. (or at a pay scale they would prefer)
Some aspects need to be monitored and/or regulated to ensure that the basic civil rights of citizens are not trampled, yes.
The "free market" gave us such lovely things as child factory labor and highly dangerous working conditions for everyone during the Industrial Revolution, for example. Issues which ultimately had to be addressed by governmental regulation since, left to their own devices, employers in the "free market" tend to seek out the highest profits, regardless the effects on the citizenry and/or the overall benefit to society.
Corporations are not "social beings" that seek to enhance their social environment beyond their own profit aims, yet recent trends often see their supporters bizarrely trying to characterize them as such, as if they have a "right to free speech", or some other rights normally accorded to human beings.
Actually it is based on some compromise between the perceived need of both the employer and employee. When the need of entity A exceeds the need of entity B, then entity B is in the position of being able to call the shots. Sometimes entity B is the employee.
What we have been discussing here is essentially how Walmart's financial and market position tends to put them much more in the position of "entity B" than might be good for the community overall.