Common sense and Craigslist if necessary. Items are valued on their current value, not the cost to replace. for example: a 3 year old sofa in good condition, purchased for $800 might be worth $200.
You should be able to estimate most items easily if you ask yourself how much you would reasonably pay for that item if you were to purchase it in it's current condition.
Look up rare or valuable items on Craiglist as suggested above.
If you are being reasonable you should not have to spend hours looking every item up.