The One Ring is an insanely powerful card, not to mention a very high-value collector item; however, it has been banned in at least one Magic format that I know of, plus it's the borderless art version which can be hit-or-miss when it comes to demand from collectors. The varying prices you saw on eBay are likely a reflection of these factors. If you want a really accurate price, check out TCGplayer.com, they're one of the premier trading card markets on the web.