WallaceB
Hello & Welcome to the Forum!
My prayer's for your father.

lt is difficult to find yourself in your situation,
with your fathers' horde to dispose of during
this time of pandemic.
There are firms that will outright purchase
or sell on consignment as well as  Auction houses
and Train dealers.
First you will need to inventory everything before
contacting anyone, so they will know what you are offering.
lf possible, look at Greenburgs' guide to get an idea of the value(retail) of the trains
. Unless you sell each item
individually, yourself,  you can expect much less.
Currently it's a buyers market & high $ values of
the 80's & 90's are history.
You can also post for sale on the various O scale train sites.
Good Luck!
Happy Rails!
dave