In Iowa I think it works like this (read here):
- The taxes are owed to the county
- There is an annual sale where people "bid" for the taxes. The "bid" is a percentage of ownership of the property that you will gain if the taxes are never paid, lowest percentage wins and pays the outstanding taxes.
- This generates a tax lien on the property and that money has to be paid back to the county before any sale can take place.
- When the taxes are paid back, the money is paid directly to the county with 2% per month (24% per year) interest. This money goes to the person that won the bid and paid the original taxes to the county in the auction.
- When the taxes are not paid back after a time you can attempt to foreclose on the house and force a sale. You receive the percentage of proceeds from the sale based on your winning bid.
Example 1: There is a $200,000 home with $5,000 in unpaid taxes. You scout the property and see that it is a well maintained home. Most likely someone simply forgot to pay their taxes or is having temporary financial trouble. You bid 25% at the auction and win and pay the county $5,000 immediately. Six months later the taxes and interest are paid to the county by the owner and you get a check from the county for $5,600 ($5,000 + 12% interest for 6 months).
Example 2: There is a home that sold for $100,000 last 5 years ago and has $5,000 in unpaid taxes. You scout the property and see that it is poorly maintained. The roof has holes in it and the paint is falling off. This could be an abandoned property. You weigh your options and decide it's worth it if you end up getting the property and bid 100%. Nobody bids lower than 100% but a few others also bid 100%, you win by random selection (?). You pay the $5,000 to the county immediately.
After the allowed period the taxes have not been paid. You do x and y and get a tax deed to the property, foreclose, and are able to sell the property for $25,000. If you had bid a low 50% at the initial auction, you would get $12,500 of that and the rest would go to the owners or their creditors.
The main problem is when taxes are unpaid and you have to foreclose. There could be squatters. They may not have had insurance and the house may have burned down. It could be a business property with chemical contamination that makes it impossible to sell. If you bid less than 100% you have to go through all the trouble of forcing the auction of the house and only get part of the proceeds (the rest going to the owners or other lien holders). You could be out all of your money. If you do get that unicorn like the $200,000 house for $5,000 but they didn't pay their taxes and you start foreclosing, they may file a court case to stop it. Eventually you may win the court case or get the $5,000 and 24% APR from them, but it could be a real hassle.
So the nicer properties with lower unpaid taxes usually end up going for very low percents because they are safe and it's almost guaranteed the taxes will be paid and you will earn 24% APR on your money. You don't have to do anything after paying at the tax sale, you just get a check from the county when they are paid.
You may be wondering why the county offers these sales. I think a lot of it is PR. Sure they get their expected money up front. For people that eventually pay their back taxes it isn't much different though and they are missing out on that 2%. But if the taxes are never paid the elected officials don't want people screaming at them for foreclosing on some poor grandma's house. Once the tax sale happens the officials can wipe their hands of the situation. It's some mean investor trying to foreclose on that grandma's house, not your county clerk who needs to get elected again.