There is none!
Check the auction sites.
www.shortyusa.com
www.berschauers.com - Free Gun auction Website gunbroker.com gunsamerica.com auctionarms.com http://www.zaffooauctionware.co.uk http://www.ongoingprofit.com/idevaffiliate/pages/201969.php
Canadian air gun supply
See the related link. This may not be the best but it's very good.
airsoft GI or airsplat (but you have to spend over 100$ to get free shiping) there wont be a site that ships every thing for free no matter what the price.
Sites such as Craigslist, eBay, or local classified is the best place to sell your gun for a low price.
You can find much good information online about guns on Youtube but you may also talk to gun store owners, gun owners, go to gun shows, or go to a shooting range and ask questions. Im sure the Colt website has a plethora of information on their firearms with exact specifications of each gun and every detail and they probably also mail out information and catalogs for free if you request it.
The website Birddog offers an extensive video library and numerous articles that Paul has written for his gun dog column. You can watch any video uploaded on the website at anytime for completely free.
If you mean where is a website that you can check to see if a gun is registered, there's not one.
Numrich has a gun parts website. Best bet for any gun parts or schematics...
On line firearm auction sites, such as gunbroker, auctionarms, and gunsamerica may represent the best estimate of what a given gun is selling for, if you search for your gun in the closed or completed auctions.