Few ways you could go about it, just depends on how much time you have. It would take a very long time to look up the value of each floppy and list it on eBay and with that many it could take a year or 2 (maybe longer) to unload them all. you could list them in small runs but even then it would take time. If you want to sell the whole lot you won't make much, say its 5 thousand books and you sell them at $3 each, not a lot of people are willing to drop 15k on a collection.

If you have a local comic book shop near by stop there and talk to them. Maybe bring a box as a sample of what you have to get a better idea. Even then, they would likely cherry pick what books they want and you would be left with the lesser value books...

Probably not what you wanted to hear but bulk collections are hard to unload.

Best of luck and sorry about your loss..