If this is a rebound copy, it's going to be pretty tricky to come up with a fair price. As the saying goes, "it's worth what someone is willing to pay for it."
That being said I did some very very very thin calculations, based on a little research. I collect Cormac McCarthy books, and as an example, his first book, 'The Orchard Keeper' is a very collectible book that came out in 1965 in two printings that totaled under 3K copies.
A fine copy in a vg+ dj runs about $2500, a good book with a good jacket runs $1500, an ex lib with markings and wear runs $800, and there's one guy who's been trying to sell a rebound ex library copy for $345 for quite some time. The $345 copy is selling for 13% of what the copy in really nice shape is selling for.
Since I can't find any of the 250 signed first Post Offices online, I extrapolated it's value from looking at various copies of Post Office, Mockingbird, The Days Run Away, and South of No North. I'm sure others here (MJP, Rekarb, Chronic) will have a much better sense of what a signed/1st of Post office runs, but based on the other books values, I'd say somewhere in the $2K range.
At the 13% for a rebound copy figured out above and a $2K value, your copy would be in the $275 range
Personally, I think rebound books are pretty challenging to call 'collectible'. The guy selling the Orchard Keeper has been try to sell at $345 for a year or two, so odds are it's 'worth' is exaggerated, which throws my whole calculation off.
Personally, I wouldn't pay much over $100 for the rebound Post Office. Your best bet is probably to sell on eBay to see what it's worth.
Others may wildly disagree, and they're probably right. You could certainly make the case that since no other from this edition are available online, it's worth a ton of money. Cheers. And yeah, post some pictures.