This is a frank word "to businessmen." Before you make an offer, first make peace with your own good sense.
Don't dump the risk on someone else
Don't say, without taking on any risk, "if we do this it'll be great, let's do it, I'll give x% once it works out." If you're ready to do business — buy the service from a marketing agency. Or else, take part of the risk yourself.
A comfort-zone owner isn't interested in leads
Recently we worked with 2–3 projects. They proposed partnership, but the leads we delivered didn't interest them at all. Because they already have "just-getting-by" sales, they're well-fed and settled into the comfort zone.
In such a partnership all the risk is dumped on the agency: you work, you show results, but the owner doesn't feel like selling. And at month's end it comes out as "the agency's fault." With such projects we didn't work more than 3 days.
Conclusion
Marketing isn't magic — it's partnership. For an agency to deliver results, the client must also sell on their side and carry part of the risk. Otherwise no lead helps.