A founder of KatLinks Roberto Robles widely uses partnerships with marketing agencies he used to compete against. When he first started, he was doing all sorts of marketing services but then he went deep into SEO. As a result, the marketing agencies he used to compete with became his friends instead of foes.
How so? When you shift your product focus and niche down other market players might become interested in cooperating with you to add up to their competitive advantages against other “generalists”.
In Roberto’s example, it turned out that not many marketing agencies were doing "deep SEO". When a customer asked for this service, the agency had no choice but to reject the request or refer a customer to someone else (and risk a possibility of losing a customer in both cases). Roberto reached out to local marketing agencies and offered a deal: if his customers wanted more than SEO he would refer them to the agency. If an agency's customer wanted SEO they would refer them to KatLinks because KatLinks is an SEO-only service, with no risk of hunting the customer. Win-win. Many agencies agreed and keep on supplying a steady deal-flow of SEO customers.