In today's Kelleher auction there were a few FDC collections, some with older better items that sold for a decent price, and others that had no interest and went unsold. Here is one that may be similar in size and scope to the one being discussed.

It opened at $375 and went unsold, no bidders. At 420 pounds, that's less than a dollar a pound, and that was too much for anyone to bite.
That said, there are people who collect them, they are pretty, especially when presented with nice cachets in fancy albums. Just have to find the right buyer for the right quantity at the right price.
Based on the volume, there are probably some better items in there. Maybe not $100 items, but likely some $5 covers like priority mail or certain sheets that may go for more. Ideally you want to find someone to take everything at once, not just pull out the good stuff and leave you with firewood. Hopefully there is a stamp club, stamp show, or dealer within driving distance to help you out.