It can be a flush valve that doesnt shut off or over flows because its not adjusted properly or a flush valve/flapper seal leaking.

As an agent I would suggest you contact owners of expired listings, those listing that have been on MLS for a long period of time - BUT DON'T talk to them about listing their properties (this may be difficult as an agent because that is what agent's are trained to do) start to think and act as a counselor, help the seller solve their problem and if you do you'll put together many transactions.Also FORGET ABOUT - the conventional way of buying real estate (20% down, using a 20-30 page contract, applying for financing and exposing yourself to intimation by lenders, underwriters and others who do not care about your financial goals and would like to see you fail in your quest for wealth) - study and understand  how to CONTROL and ROLL real estate - C&R is the highest real estate leverage ever created - C&R is about selling (assigning) real estate you don't own. it all boils down to creating a "skinny contract" with only one contingency and offering that property (contract) up for assignment using the public auction method of marketing.You can do it - flush your mind of the conventional ways of doing the real estate business and start to turn over rocks to find a sellers with a need - help them and your skills will help you make some serious money.Oh!

There were plumbing issues (the first floor tenant's toilet rained in the basement every time she flushed it).

I want to comment you need to blame yourself-  who buys something in who knows where without flushing a toilet or seeing the quality of people walking the streets.Yes you can tell the quality of people just by looking at them.   

As a young worker, we are often flush with cash but with no time to spend it.

When in the bathroom I'll flush the toilet right there.  

To flush out private or side agreements or other details. 

We have three relatives visit us every year and I can see a substantial increase in our water and sewage bill even thought they only stay with use for a week or two due to the extra baths, cooking and toilet use.Washing cars or neighbors stealing water from a faucet can also be a problem in certain areas.If you can find a good time to check then check the meter on the street, make sure there is no water running in the unit and that no one will turn on or flush anything at all, let it sit 30 minutes or so and see if the meter goes up which would indicate a leak somewhere.The tenant may also be able to call the water company and plead their case (Not blaming you) that there was a leak that was out of their control and that they didn't realize it soon enough and see if they will give them a break on the bill, it depends on the water company but I have seen the water companies in my area give a break on the bill for certain circumstances.

I've done the yellow letter campaign thing for the last year and am looking for other effective means of flushing out sellers.

You can and will blow through $5k in Google AdWords or a Direct Mail Campaign and get absolutely nothing to show for it ::toilet flush::.