If she lives in a rural area then an estate auction is the way to go.
If she lives in a non-rural area than an estate tag sale is the way to go.
If the father-in-law was a big collector of specific items (books, glassware, etc) then try to see if there is collector out there that would buy the lot of collectables.
I’ve never heard of an estate tag sale, so, that’s good. Something to look up. Thanks.