You want to do something effectively... And spend no money doing it?

That's not only impossible but it is the wrong way to think. It will cost you much more money in the long run.

Marketing is an INVESTMENT. Not an expense. You are among for the best ROI, not the cheapest initial cost.

The cheapest initial cost is often the WORST ROI (normally costing much more down the line).

I don't have an answer but do understand the question since a lot of people would love to have this kind of access to Craigslist.

Follow-up question: If you pay someone to enter these manually (say someone working minimum wage), does that conflict with Craigslist's terms of service? If not, that might be a possibility.

Doing this is a violation of Craigslist's TOS as it is considered spam --- which it is. If you want to advertise, you must pay. End of lecture.