The success of your preaching will be substantiated by how many books are sold. Anything you want to sell, you have to a little canvass, so he gives some money for the service of Krsna. That is his good luck and he gets the chance to read some transcendental knowledge. But if you only irritate and he goes away, that is your less intelligence. The fact is that we have to adopt the same tactics as ordinary salesmen adopt, but the difference is we do it for the satisfaction of Krsna, they do it for sense gratification.
Letter to Bali Mardan, 30 September, 1972