First of all, Merry Christmas first to all theof my friends here.

a lot more natural, although your wording wasn't wrong

I clearly remember a Christmas eEve day when I went on the street with my then-girlfriend about in 2005.

We planned to sell some Christmas accessories like LED lights, Christmas hats, and sparklers onlyjust for fun.

"only for fun" is not grammatically wrong, but it's a lot more natural to say "just for fun"

Unfortunately, we sold nodidn't sell anything out, so we used partsome of them ourselves and left the rest back home.

First, Merry Christmas first to all theof my friends here.

Your sentence is correct but this is more how'd we say it in the US.

In fact, I nearly forgeot that it'll be Christmas Day tomorrow, which was a long dreamingy day when I was in 20thmy 20s.

I'm not sure what you mean by "long dreaming day" so this is my best guess.

I clearly remember a Christmas eEve day when Iaround 2005 whent on the street with my then- girlfriend about in 2005nd I went out.

Unfortunately, we sold nothing out, so we used partsome of them ourselves and left the rest back home.

The next year, I gave some of the rest Christmas accessories to another girl.

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The next year, I gave some of the remaining Christmas accessories to another girl.

First, Merry Christmas first to all themy friends here.

In fact, I nearly forgeot it'lld be Christmas Day tomorrow, which was a [long dreaming day when I was in 20th].

“A long dreaming day” sounds like you spent the entire day dreaming. Is this what you mean?

“20th” usually means “the 20th day of the month,” but I doubt this is what you mean.

I clearly remember a Christmas eEve day around 2005 when I went on the street with my then-girlfriend about in 2005.

Unfortunately, we sold nothing out, so we used partsome of them ourselves and left the rest back (at) home.

“Sold out” means “卖光”, so “sold nothing out” implies that you sold many things, but you still had some of each item left unsold. Is this what you mean? Or do you mean that you didn’t sell anything at all (卖不出去)?

The next year, I gave some of the restmaining Christmas accessories to another girl.