The quote is from a book called "Scientific Advertising" by Claude Hopkins, from early in the 20th century.
This is the definition of "individuality" that fits here:
"The individuality of a person or thing consists of the qualities that make them different from other people or things."
The purpose of advertising is to convince people to buy one product rather than another. One way to do that is by portraying the person advertising the product with an individuality attractive to the intended audience. The book goes on to describe some ways of doing that:
Google books "Scientific Advertising"
"One man appears rugged and honest in a line where rugged honesty counts. One may be a good fellow where choice is a matter of favor. In other lines the man stands out by impressing himself as an authority. We have already cited a case where a woman made a great success in selling clothing to girls, solely through a created personality which won."