I did a google search for United Tax Liens and Tax Lien Wealth Builders to checkout how their online presence is and a link to this post popped up. 

If you are lucky you are buying a mom-and-pop landlord property that has been owned and operated for a long time.

Several things to contribute here: 1) Private Lenders with no credit experience emerge doing down payment and 100%/High LTV financing at the end of every cycle, 2) they're among the first to loose their shirts in every downturn, 3) although someone termed this type of lending "gator", it's not new, 4) the guy that coined it didn't get into real estate untill well after the last crash, 5) he has no real credit background or experience (he's never been a lender), 6) Google the guy's name and "looses professional license in AZ" to see what pops up, and finally 7) he's really effective at telling people what they want to hear, filling their heads full of hopes and dreams, getting them to buy his training, and his minios generally won't want to hear what a real, seasoned lending vet will try to tell them.

For a Mom and Pop with just a few, there's no benefit.

I think a company that would arrange to put a used, older, reasonably remodeled home on to a vacant lot on my park to sell in place would be very successful- particularly for us 'mom and pop' operators.  

Then, we get popped with a bill for $15,000 for taxes, book keeping, etc.

PV does not have as many mom-and-pop shops as the town went from zero to bursting fairly quickly so it is loaded with big box and chain stores. 

We have very good data about investors in Milwaukee, the vast majority is mom&pops.

I not going to post links here... but a simple google search will pop some alternatives.I've not used the calculator here and I don't see information on what it covers or what questions it asks or what kinds of results it gives.

The other site that ends in /landlordbookbonus (which is where the audio book said go) downloads the material but an error messages pops up when you try to open.