That's because they're not traded very often and unfortunately we don't have them on hand to sell to make a more accurate value.
However, as we have such an extensive database of values we are bound to need some help if a certain range, or type aren't the ones we tend to see quite often so thank you for suggesting some values.
With the Yellow Spaceship Door though, you say that they've 'apparently' sold for 60 Gold Bars, and that we're 1200 Credits out - we can't just change values on assumptions for most cases, but we can try work out a value that would be fair *if* those have already sold for that amount. Continue to keep updating us with values though, especially those traded less and less nowadays Tom. I'm not on my laptop currently so I haven't been able to change some yet, but again thank you all for helping out!