I'd go back to Egypt and trade for Pharao Tuts dagger made from meteor metal. In return I'd give him a gas station tactical all black heavily serrated assisted folder which never needs sharpening and has lots of skulls on it.
He would think he got the better end of the deal and would have trouble holding back his grin and me too.
Win win.
Also even if the dagger would be carbon dated to be from our time (since it travelled through time and didn't age) and thus be worthless for a museum or collector it's still a cool piece of art for me personally.
I know that this is for fun, but as an archaeologist, I just had to chime in...
You can't carbon date metal. You can only carbon date things that were alive, because what you are dating is the ratio of (radioactive) C14 to (stable) C13. While organisms are alive they are constantly taking in C14 from the atmosphere and things they eat at a fairly stable rate, and then those isotopes are incorporated into their cells. As soon as a thing dies it stops making new cells and the C14 in the material starts to decay into stable C13. The half life of C14 is less than 6,000 years (if I recall correctly, I don't work in an AMS dating lab myself), which is why you can only carbon date things to a maximum age of around 50,000 years ago. But that means that you can only date things that were alive.
So unless your knife that you brought back had bone or wood handles, it couldn't be dated (plastic is made from fossil carbon, which was alive, but hundreds of millions of years ago, so is composed of entirely stable carbon). But, just to make things more complicated, trees only replace cells in the outer layers, so the inside of an old tree can date to hundreds of years before the outer part (this is called the "old wood" problem in carbon dating).
But if your knife had wood handles, and you brought it back, the carbon inside would decay normally, and the carbon date would be the same as if it had started from back then. But if you had a knife with handles made from the inside of a 1000 year old tree, and brought them back to 1330BC, and traded with King Tut, then the knife would carbon date to 4280 years before present (because "present" in carbon dating is always 1950...).
But just to add extra things to think about, the amount of C14 in the atmosphere back in the mid-20th century was way higher than average, because most C14 is made by solar radiation, but all of the nuclear testing and explosions of the 20th century made lots of new man-made C14. But these days modern things are actually testing older than they are because of all of the fossil carbon being pumped into he atmosphere by fossil fuels like gasoline and coal. When we were blowing up lots of nukes we were putting enough radioactive carbon into the air to counteract the fossil fuels we were using, but now that we are burning more fossil fuels and blowing up fewer nukes we are throwing off the carbon ratios...
Okay. I'm sorry about the digression.
Time travel is hard.
I would go back to 2007 and buy a bunch of Lone Wolf Knives. But those aren't traditional. So if I went for a traditional, I would invest in bulletproof gear, then go and pick up a V-44 survival knife from WWII. Because I think they are cool. One was donated to the museum I worked at, and I had to research what it was. Marines in the pacific theater (early on) used to ditch their daggers for the Air Force Bailout Kit V-44 "machete". The V-44 big bowie style blade was a lot more useful in the islands. Of course the Marines switched to Ka-Bars really quickly, and the bailout kit knife changed, but those V-44's were cool. And I like the idea of Marines in the Pacific fighting with bowie knives.
http://www.quanonline.com/military/military_reference/fighting_knives/survivalhistory.php
