Definition of "clean" items.

As there is dispute sometimes on definitions of "clean" for a item, I'll make this little guide.

A item is "clean" when:
- the item was not gifted (no "gifted by xxxx" text on it)
- the item is tradable
- the item is craftable
- The item has no (invisible) craft number.

On craft numbers, up to craft #100 is visible in-gaming, but on backpack viewers like backpack.tf they can show in the thousands. Both are valid arguments for being not clean. There are traders that search certain numbers that are higher then #100 that specifically search them (see footnote).

The following properties to items do not account for being "dirty":
- Painted (any color)
- Renamed.
- Description changed.
- A set "style" (on items that have styles)
Because all those can be individually removed (or in case of the style: changed) by a owner of a item in-game.

Special cases:
Those are items that ONLY can exchange hands by using gift wraps, as there is no choice on those items then to gift wrap, its not applied to those items to be "dirty".

As you can see, all these properties apply to the specific item, without taking into account external things, like a duped item. Duped is not a property of the item itself, only that another item exists with the same original ID. Therefore a duped item can be "clean".

Resources:
http://wiki.teamfortress.com/wiki/Crafting#Item_numbering