I'm planning on launching a directory site (you might also call it an industry vertical) where companies will have to pay in order to be listed. You could also call it an industry vertical.

I'm planning to do the payment entirely based on a Pay-Per-Click model, meaning creating your directory entry is free, but if a visitor clicks on your entry you pay.

So I'm looking for an existing pay-per-click engine or adserver for this; I don't want to do it myself. Advertisers should be a able to create, manage and delete their entries entirely in self-service. Cost-per-click should ideally be managed through an auction.

I've started searching around using terms like "ppc solution", "ppc engine" or "adserver", but haven't found much.

Am I searching for the wrong keywords? What would you suggest? Do you know anything that comes close to it?

Every hint for a solution which might do part of the job is also appreciated. I'm not generally opposed to doing some development, maybe based on an API.