Depending on the website, a keyword search could lead you to ads like this. On Ancestry, I found an ad for a saloon that was originally owned by my 2nd-great-grandfather and then passed to his daughter, Kate Nehren. In the 1889 publication, Our Firemen: The History of the Pittsburgh Fire Department, from the Village Period until the Present Time, her name is on the ad as the proprietor so a keyword search of "Nehren" found it.
In other cases, you may have to browse every page of a local publication. When using the wonderful resource Historic Pittsburgh, a keyword search of the 1889/1890 city directory did not result in the Moorhead ad shown above. There were 52 "hits" within the publication for Moorhead, but none of them was this ad. So if you rely only on search results, you could be missing a great piece of information about your ancestor.
Whether you try keyword searches or browse local publications from your ancestor's town and time period, it's worth the effort to look for ads for any family member who owned their own business. You never know what you may find!