investment return, also, Alabama has no limit, you can charge 50% interest,
These certs are auction off periodically, call the local muni govt they shall give you a list of outstanding liens,
I know Delaware is hot right now since their municipalities are about to go down in debt.
Two things keep in mind,
Be prepared to pay legal fee which will cut to your return to zero in case of defaulting
pick Liens on People that has intention to pay, call them interview them, reason with them, better yet work out a payment plan.
do read my first post, it contains most of the technical details, btw, these certs are not endorsed by Canadian govt, so you watch out for that.