I'm no expert but I if i can make a few suggestions to get you started with the process. You might want to pick up a copy of Greenbergs pocket price guide to Lionel trains 2025 edition.  You can get it in a lot of places. It will give you an Idea of about how much certain items are worth.
 I think that selling them to the right people is the key. Things like condition and rarity are always the big factor. Some places just buy collections bulk and are looking to get them as cheap as possible for resale. Collectors might pay a bit more per item but you will have to connect with them individually and it will take more time to sell them all.  Search the internet about how to go about selling Lionel collections and see recommendations on what to do by people who have done it.  Hope it turns out well for you.