Does it have the "fix"? If it is guaranteed working and has the fix you may get a crazy amount for it, like $800. I'd say just eBay it. You will get at least market value and you may even hit the lottery.
Selling direct through forums is tough, hit or miss at best. At the very least you should get at least $500 based on rarity of a working AmigaOne.
-Edit-
Now I see you are thinking about buying it. Value is in the eye of the beholder. I wouldn't buy an AmigaOne at any price, too much risk of it dying right quick for my taste. Others (historically) would likely bid it up to crazy lofty amounts (possibly $800+ on eBay) just to be able to run OS4.. I say historically because they have not been on eBay for a while afaik. What people would pay a few years ago, we could only guess if they would now.
Another question is if the OS4 CD is legal and registered and who is it legally registered to? They would need to be willing to jump through hoops to get that registration transfered to the new owner.
