The most I've paid for fabric at a thrift store or yard sale was $2 per yard. And that was for newer fabric.
Guess if I had the opportunity to purchase such a large stash of fabric... I would give them a
"if I buy it all" quote. Yes, they can piece it out so to speak, however it will take time and energy and they will still be dealing with whatever doesn't sell. It's to their advantage to sell it all in one lot and be done with it.
A friend shared some of her Mother's fabric with me. Spent days washing it to see what was salvagable. Then, after deciding what I could and would use... I gave the rest away.
Let us know what happens.