To donate, I would give it to my quilt club or post it on freecycle where I've picked up fabric for free (look online, freecycle is local to everywhere).
To sell, I don't know other than ebay. Does etsy allow selling?
I understand about downsizing. For Christmas, my niece & boyfriend gave me 4 laundry hampers of his gramma's fabric that the family had to give away when she died. It took me all January to sort, choose and fold into my stacked-to-ceiling plastic drawers. I only allow myself the amount of fabric that fits in those. After folding properly, I ended up only giving my quilt club about 1/3 of one hamper. We are always happy to be given free fabric.
I've already instructed my family that when I die, to look through my stash, take what they want, then donate the rest to my quilting club where we are all seniors with limited income.
What I learned in January was that by folding fabric like at quilt show booths, I can fit in twice as much as I did before. You may find that to be true for you too.