Okay so i've heard that you can get your armor back by going back going to where you first found it but do you also get the sword back if you've sold it.

If you guys know anything about it let me know please thanks in advance and merry Christmas.

You should search at TES:wikia.But i think that there is only 1 nightgale blade in the game.If you are playing on Pc you should use Console commands

Why would you sell the armor?

How cool is the Nightingale armor? I haven't seen it yet, but i'm almost to the point in the Thieves Guild to get it. I have the blade now.

I did it by accident I was selling a bunch of stuff and accidently sold it but I dont want to buy it back for more than I sold it for

The Nightingale armor can be replaced if lost, Just go back and touch the same stone that she tells you from before. Reports say that for some people it does not work for some reason.The sword can only be obtained once.
[img]http://images.wikia.com/elderscrolls/images/c/c2/Nightingale_Armor_%28Male%29.jpg[/img]
Information from wikia.

Steal it back?

Its awesome it reminds of a medieval batman costume. I don't regret doing the thieves guild first because this armor looks sick and has some nice perks.

Well some Items stay in the Inventory of the Mechant where you sold it. But i think if it isn't or wasn't a Quest relevant Item it will dissapper after 2 ingame days. And will be back some days later. I sold a Ghostblade which stayed in his inventar a few days. As i looked after 3 days it was gone. Then a few ingame days later it was there again. So i think the Inventar is randomly. I also sold the Staff of Magnus which is still in the inventory of a Merchant in Rifton.