Maybe the house you offered on is in better condition than the average trashed out bank repo, or has some feature that housevlaue.com didn't figure in.
I think the websites are pretty amazing, they come with in 10% on a regular basis. pretty good for a computer chip.
I am sorry to hear that you are having second thoughts regarding the offer you made on a property. Are you working with an agent who is familiar with the area where the house is located?
There are lots of websites that claim to be able to predict the value of property, sight unseen. As I have said in the past, doing an estimate of value is an art rather than a science. These websites use a series of complex algorithms and fancy mathematical equations to predict the value of a property...often times, they use "comparable" property sales that are in areas that are just not relevant to yours. If your property in question were truly in Colonial Village (which is the area Trulia indicates), it is very possible that the site you used based the value of your property on recent sales in nearby Oak Park, which is not an apples to apples comparison.
Also, these equations do not take into consideration any upgrades, features, amenities, choice locations, etc. On the other side of the coin, they don't factor in negative variables either (being on a main road, leaking roof, abandoned house next door, etc.).
Since none of us on this forum know anything about the property in question, it is difficult to pass a lot of judgement that the value presented by this site is valid or not. My suggestion, much like some of the others, is to find out what the recent, relevant sales are and determine if your offered price is in line. Also, if you are getting a loan for your property, your lender will send out a licensed appraiser to determine if the value of your property is adequate to secure the loan you obtain.
Good luck to you...
Since you have already submitted an offer, the next step would be to ask your real estate agent to give you a list of comparable sales in the neighborhood -- which you probably should have received before making an offer. Otherwise, how did you come up with a price? You can't use existing inventory to price from. You need to look at sold comparable sales and also the pending sales -- because pending will show you where the market is moving. Pending sales won't give you the actual "sold" numbers because they are still in escrow, and that information, unless your agent can squeeze it out of the listing agent, is not available, plus it is sometimes confidential. But you can look at the list prices of the pending to figure out a median.
On top of everything else, you will pay for an appraisal, unless you are paying cash. The appraisal will show you an accurate opinion of value. If it doesn't appraise for the sales price, depending on the contingency clauses in your contract, you can cancel.
But please do not rely on online web sites to disclose value. If they were accurate, you wouldn't need an appraisal and banks would not require an appraisal. :)
I saw one website for my area recently that lists the median prices of homes as over $1M. I know that they are selling for the $800,000 range, so their numbers are based on I don't know what.
Your local Board of Realtors will often have numbers on tehir website, which tend to be a lot more reliable.
If you have an appraisal contingency in your purchase agreement, that will allow a renegotiation of the price after appraisal if the appraisal comes is lower than your offer.