There are thousands of missing items on the 'steam gift' category, and, I mean thousands. Puddle, Stacking, Blood Knights, Lost Planet, Towns, Redshirt, PixelJunk, etc...

Any reason for this? If I had to take a guess, games are only indexed once someone trades it with a bot, why not just index every steam item? Granted, I've never worked with the item API, but, I'm sure there's a list somewhere? Or someone willing to provide you with the list of all the items?

You would get a list of thousands of items. Would take forever to load and just stall your browser to display them all at once in the search. Would be neat if you could putt a filter and then search; so you only end up with the game you like :)

Wouldn't it be worth implementing a spider, if at the very least just to scan member's of this site's inventory? Even more so, everyone's inventory on Steam?

Not sure, that may break some Steam API ToS agreements.