[yummyfajitas]

In any field in which there is a concentration of employers relative to employees. Which is to say: most of them.

This is true not just of the employment market, but of most goods markets. There are far more buyers of cars/computers/cheese than sellers. Does this mean that pizza producers have oligopoly selling power?

Your theory is too broad. It applies to virtually everything.

In the economy at large, the situation still remains true. An employer need pay an employee no more than that employee could claim at another job (or by going into business for him or herself), given the employee's skillset.

Conversely, an employee needs to accept as wages no less than the wage he could get from another employer. This is true of any market - a purchaser needs to pay no more than market price, and a buyer needs to sell for no less than market price.

All you are describing is market pricing. Are you claiming all markets are oligopolies or oligopsonies?

[dredmorbius]

The distinction between employment and sellers markets is that an employee typically has a (mostly) exclusive relationship with a single employer, and relatively high switching costs (interviews, unemployment), particularly for more advanced / skilled professions.

A buyer usually has very low switching costs between merchants. That said, yes, there is a great deal of concentration in retail, especially as you go back up the supply chain. For electronics and other advanced goods, there is typically one or a very small set of manufacturers (at least at the component, if not the finished product scale), e.g.: Foxconn for laptops and mobile devices, a handful of disk drive and memory manufacturers, etc. For food, there's a huge level of concentration at the mid-market, through Monsanto, Tyson, Cargil, Con-Agra, etc., despite the huge number of individual food outlets.

While I don't claim that all markets are oligopolies/oligopsonies, a great many are (or exhibit a great deal of concentration, or of market distortions such as healthcare) including many of those comprising a large share of the US economy: health, finance and insurance, utilities, retail. Enough so that what we consider to be the conditions of a free market: economically small buyers and sellers with equivalent information and low switching costs meeting in an open marketplace, are actually met in only a small portion of the economy as a whole.