Thank you all for the good advice! It sounds like it might be better to sell. Do you know if people will buy it for parts or might buy and repair themselves?
Yes, there are camera repair people with parts who will either repair it or use what's working for parts. They can replace a bad shutter for less than $10. Lots of AS-IS cameras are sold on ebay. Don't bother with Craigslist.
The camera apparently has many good parts that are valuable, every piece is sold on ebay, a main board gors for $200, for example.
Expect to get between $300 and $450 depending on condition, so tale lots of good clear photos of the exterior and flash foot, describe any physical flaws and the error 20. Don't guess at the issue unless you know, you can get a unhappy customer if you guess wrong.
Finally, if you spend $15 and send it to canon in a flat rate priority mail box, they will tell you what is wrong and give their estimate. At that time, they may offer to sell you a discounted refurb, but you will not save $300 over their refurb sale prices.