Optimize for word of mouth marketing

Start here.

Make it highly likely that your customers will tell other potential customers about you.

Hypothetically, this could be all the marketing you require. But don't do it just because it might mean you can Cut your ads budgetCut your ads budget
Just to see what happens!

Maybe, like eBay, you'll find the answer is "not much."

Remember the Parable of the Pizzeria - are your ads creating new sales, or just claiming credit for sales that we...

Do it because it forces you to Talk to customers Private or Broken Links
The page you're looking for is either not available or private!
, and talking to customers is fun.

Also because increasing word of mouth marketing is a way to Seek win win winsSeek win win wins
If a win is good (it might not be), win win is better, and win win win is better still.

Examples of win win win:

Making a mutually beneficial introduction
Recommending a book / product someo...

When you've succeeded, you've created a situation where customers gain social status (and maybe some kind of bonus) by recommending you, someone else gets their decision process simplified and their problem solved by you, and you get more customers. Win win win.