Duplicated items are items that are restored by steam support when a backpack is hijacked (some past glitches exist, but are patched), making two of the same item exist. There are no rules against selling those items.

As for the person asking for a refund, I'd wait on a mod to answer, but it was up to him to research if he wanted to make the deal, not you.

the id of the bud i sold is 1530971129. if someone can tell me if its dupped

No, they don't ban the ones that sell duplicated items. I remember that i sold a "possible" duplicated Dashin'. Never knew that it was a "suspectible dupe" until i checked the hat on one of the profiles on backpack tf(the guy whom i sold it to). Got abit scared at first, but i could not find any proof that it was dupped. But i know that this link right here http://www.tf2outpost.com/trade/7931616 shows a dude who sells the "dirtiest fucker" ever. It is dupped, gifted. And the story is very long. Hell, even JON had this in possesion. The user did not get banned, but i do know that Outpost do ban most itemduplicators. But please, make sure to be aware about links to outpost, Sneeza recently told us that there is a fraud replica of outpost. Spread the word.

Careful he might give you duped buds back for your non duped buds. Someone tried that with me. Then said he'd report me to Steam if I didn't take them back. I just blocked him. My level 23 buds were not level 47.

Threatening others with dupes i hear? Tragic.