Well, first off, my database shows a birthday (12/7/76) for that bass, so it's likely that someone asked here about the build record. A quick search on the number, though, comes up empty.
In any case, here's what I have in the sold on eBay for data, for S1 basses:
1974 - $1601
1976 - $3363
1976 - $2426
1981 - $4700
1981 - $2425
1985 - $2299.99
In the failed to sell on eBay data:
1977 - $2425, reserve not met
So, a range of $1601 to $4700, with an average of $2802, but a median of $2426. Splitting the difference on the two '76 basses I show (neither one is 569, btw.), gives $2895.
My feeling is that the $2895 number is a good reference, IF the bass really is in excellent or better condition. Temper that with the possibility that it may need an electronics upgrade, which runs a few grand. It might not, though...
The $3k Buy Now price suggests that he's looking for as much over $2500 as he can squeeze out of it, and so might even let it go for as little as $2250. But with three offers pending, it's not likely to go for less than $2750, and will probably actually sell (if the offer goes that high) for about $2850-2900.
So, if _this specific bass_ is worth an extra $150-250 to you to freeze out any other buyer, bite the bullet and pay the $3k. If you're not in a hurry, pass on it and see if it comes up again.
If none of the three offers is high enough, the seller may well drop the Buy Now price by a few hundred and try it again. At that point, the Buy Now price would be just about at the instrument's value, and you could buy it for what it's worth.
One last datapoint: since February, there have been nine separate S1 basses available via eBay. Six have sold; two haven't; one is up now. So they come by about once every three weeks or so.
All that said, remember to check the dealers. Ferinstance, Bass Central has a used Small Body, Short Scale in Buckeye Burl, with Europa electronics for $2999. Or, if you're looking for the Standard body, a used Mark King Sig. Standard in Cocobolo for $3499 (with an Omega and LEDs, no less!). Either one would be a reasonable alternative to the S1 on eBay, and they've both got major Yum-factor woods.
Now, neither is a Series bass, and neither is as old as a '76. And, if you're realistically looking in the $2250-2500 range, they're expensive and you should probably keep sifting eBay listings until you fine the combo you want.
But if you're looking for an eye-popping Alembic in an old style body shape, and can take a slightly less comprehensive electronics package (the basic sound of the instrument will be essentially the same or even better because of the woods), and can handle $3k or so, there are lots of places besides eBay to look.