who will take a commission/percentage to sell your collection.
It can take months or even years to liquidate a collection like your mother's. The advantage of having a re-seller is that the dolls are out of your hair and you will get the best possible value, even allowing for the re-seller's cut. (You could post here and at other doll boards asking for a re-seller with references.)
Yes, you could try to find a buyer for the entire collection but 1) you're unlikely to find one buyer for it all (as Kelly noted), and 2) you will get pennies on the dollar for what the collection is worth individually or in small groups.
That is an amazing collection, and, if you are in a position to take your time, you can realize the most value from selling it. Best of luck!