If I have to sell my services .....

........ instead of saying how good I am, I'll rather say how shitty others are. When you address a problem that people always knew existed, but couldn't do much about it, they have all ears. What you are basically doing is confirming their opinion or bias.

This is the same tactic political parties use. Instead of saying what good they did when they were in the government, they say what bad their opposition is doing in the government. The math is quite simple here, this way you basically stir the water to create drama and conflict, and who doesn't like to witness drama and conflict?

But, and it's a big but. When you bring someone down to market yourself up, you better deliver on the points which you criticised about them. Else, it'll be just a matter of time before people will lose fath on you.