Do you have to be a resident of the state/city you are buying them in?
I suspect not in most states. You do not have to be in Maryland.
How much cash do you need to get started?
I started with a $3500 lien and spent another $7K in assorted expenses. I got the house and resold it for $40K without doing anything to it. That was a home run. Don't expect that to happen often but is can and does happen. It shows you can start with a modest amount in this game.
What are the possible ways you can get screwed over?
1)The Number one risk is not Knowing the rules and how they affect you.
2) The next biggest risk is bidding too much.
3) In Baltimore where I bid the city can screw things up.- Voided liens get lower interest, receivership can wipe out a tax lien.
4) Deterioration of vacant properties
5) fire for vacant or uninsured properties
6) Bankruptcy tying up you money an possibly reduced interest.
7) federal tax liens
I have had problems with 2-4, never had an issue with 5-7.