Wow, is that a loaded question or what.
Your questions would take several pages to answer more fully but, yes, one can find a relative value and sell them online as an individual.
Modern guns shipped to FFLs only of course.
Realizing anything approaching full value takes a LOT of work for the seller. This is why a local dealer's offer will seem low.
To name a few things the seller faces, proper ways for a new seller to price items so that they sell quickly, learning to precisely describe and list items to maximize exposure, learning to photograph the items so that the listing will not result in a deluge of requests for better and more precise photos, and, most importantly, building a record of positive feedback quickly.
Nothing will kill the value of a sale quicker than listing items with negative feedback for a new seller.
I am sorry to hear of the loss of your friend. I have no way of knowing the level of your friendship with the widow.
I advise that if you are to become involved in the liquidation directly, you should have her directly involved as well in all phases so that she has a complete understanding of the process. Failure to do so often leads to problems.
JT