A business that is unionized is often not viewed positively by prospective business buyers, and will usually limit the number of buyers who will consider acquiring the business.
Nevertheless, if your employees have organized there may be benefits that the union offers, that could be positioned as positive attributes to a prospective buyer. For example, in many construction trades the union will do skills training, certification, and safety training. Also, while in poor economies a unionized company may have a difficult time being price competitive, in a strong growing economy it may be easier to attract staff for higher-paid union jobs, thus allowing a company to take advantage of more growth opportunities. Another consideration is who the customers of the business are and their preference, or even requirement, for using union labor. There are some industries in which a non-unionized labor force will limit business opportunity due to customer preferences for using unionized vendors. If your business' customers require or prefer union labor, your company may be perceived more positively by some buyers given the competitive barrier this provides against non-union businesses.