Ebay sold is the way to go, but you have to pay to use that information. User submitted prices only goes so far and typically won't give you the accurate value. That is because aside from a new release item, most collectors aren't going to buy something at the market value, thus user submitted pricing will undervalue everything. I've seen this happen on other websites, and is also the reason why I don't bother with inputting that information anymore, nor trust the valuation a site may give me based on those numbers.
There is also the case where the market value can change on an item and then the user submitted prices that already exist become incorrect. One example I can give on a specific Transformer I have. On a website that used Ebay sold data, the value of the item was $800. When the site stopped using that data and switched to user submitted prices, the value went down to $500. Then a re-issue came out and the demand for the item decreased and while the value on the website did not change, the actual market value fell to around $350. Actual current market value is ~215 and my original price paid was $125.
So basically, you can only get a real value of an item on a site like this if you limit the values in the past x amount of time. That means you can buy something and have a known value for say, 3-6 months. Then if no one has gotten that item, or they put it into their collection but do not put a price (or get it for free or some bargain) then the value you'd see would be way down or maybe even unknown or zero. As cumbersome as it is, the best way to determine a value is current market data and not user submitted price data.
