If we are talking about an item that is not being manufactured anymore, and is only selling on the secondary market (that is, not being sold any longer by stores as an item in production at a retail price), the best way to determine its value is to see what prices that item has sold for on the secondary market. And ebay is probably the single biggest secondary marketplace in the world, so seeing what the item has been selling for on ebay is the perfect way to determine its value. You look at several sales, take an average of the price the item sold for, and you have that item's current value on the market.
Does that mean that amount is etched in stone? No, prices will fluctuate over time... but taking an average of several prior sales of that item will give you the best possible idea of what that particular item is worth on the market today.