I am a vegetarian. I’d rather have cast iron that has never had animal fat in it so I just buy new. However, if you are not worried about the oils that have been in your pan you may be lucky enough to find a nice piece at a thrift store, yard sale, or on Craigslist. However, sometimes used pieces are sold at premium prices because people value years, sometimes decades, of seasoning.
I did buy a 7 quart dutch oven off craigslist one time. It had only been used for one Thanksgiving dinner and I decided that I could just wash it out really well. The price was too good to pass it up. If, by chance, you live in Tucson and a few years ago you sold a 7 quart cast iron dutch oven to a woman that met you in a bowling alley, I’m still using that piece.
If you’re at a yardsale or thrift store and find an old, rusted piece of cast iron cookware that does not have any cracks you can restore it. I’ve not done that so I’m not going to describe the process. I would get on the internet, research it, then tell you what I thought I read. My recommendation is to just research it yourself and give it a try.
My one suggestion about used cast iron is to be careful of the manufacturer. Make sure it is made by a company whose materials you trust.