In line with what @Wayne Brooks and @Jay Hinrichs referenced, I had a mentor that purchased several hundred thousand dollars (literally) in tax liens several years ago. We would help him grab up liens on the secondary auction every year for a couple years. But then the hedge funds found out our dirty little secret and started bidding them down to ridiculous rates (i.e. 0.25%). He went front buying $250k in liens to buying essentially $0 in liens. Not worth it anymore. And like Jay mentioned, most of the properties left at decent rates are duds. 

To be fair though, it was great while it lasted.....

His strategy would be to buy liens on properties that hadn't had lien in the past, and if they did have some that they were paid off fairly timely... only owner-occupied homes... and other criteria that gave him a some level of confidence that they would be paid off (he wanted the 18% interest, not the property).