I went to a varietycrafts store to buy some stuffs to make a mask for Halloween etc.

There were many products for Halloween.

Your sentence is correct but here are US alternatives.

- There were a lot of products for Halloween.
- There were lots of products for Halloween.
- There were plenty of products for Halloween.

I bought some, and I went to another store.

The store had already lesssold a lot of Halloween merchandise, s. Surprisingly, they hadwere already selling Christmas merchandise.

へー！

I plan to participate in the Halloween event at the Night Museum.

If the reader doesn't already know about "the Halloween event", then you need to write 'a Halloween event' instead.

I went to a variety store to buy some stuffs to make a mask for Halloween etc.

The singular form of `stuff` when used as a noun, is the same as it's plural spelling. When you spell the word as "stuffs" the person reading is thinking that you mean the verb form, for example "He stuffs his clothes into the suitcase".

There were many products for Halloween.Halloween products,

The original version of the sentence is fine but I converted it to use `Halloween` as a [noun adjunct], because "Halloween products" makes it sound more natural.

To sound fully natural we need to join it with the next sentence, thus I removed your period.

I bought some, and I went to another store.

Now that this (sentence fragment) line is part of the previous sentence, we have to drop the comma. In effect, we have combined the "I bought some", and the "went to another store" clauses into a single one. Now the single clause can be used as a single subordinate clause of the previous line.

The next store had already less Halloween merchandise, surprisingly they had already begun carrying Christmas merchandise.

Since there are two `stores` in your story, we have to better distinguish between them. When you open the sentence with "the store", the reader feels some initial ambiguity as to whether you're referencing the first or the second store in your trip.

I'm afraid I can't explain the other corrections of this sentence very well because they are beyond my understanding of English grammar. I think that using "already" requires that a verb or verbal phrase precede/proceed it, something to that effect.

Isn't it too early for this?

You have an incomplete sentence here, every sentence requires at least a subject and a predicate. My correction is very colloquial sounding but still natural and, within it, "for this" functions as the subject.

I plan to participate in the Halloween event at the Night Museum.

Is “Night Museum” the name of the museum? Or do you mean a museum with a different name is holding a “Night At The Museum” event?

I went to a variety store to buy some stuffs to make a mask for Halloween etc.

There were many products for Halloween.

"Halloween products" is more natural

The store had already less Halloween merchandise, surprisingly they had already had Christmas merchandise.

Unsure of meaning here - if you mean generally the store didn't have much merchandise then "The store had less Halloween merchandise" is better. If you mean the store had less merchandise because people had already come to the store to buy the Halloween merchandise then it's better to say "the store didn't have much Halloween merchandise left"

I went to a variety store to buy some stuffs to make a mask, etc. for Halloween etc.

Etc. means "and other things" (literally, in Latin) and I moved it to indicate "a mask (and other things)" rather than "Halloween (and other things, i.e. other holidays)" which is what it sounded like in its previous placement

I bought some, and then I went to another store.

Helps to clarify that this is what you did immediately after in the context of the story, ultimately stylistic though

The store had(was already lessrunning low on Halloween merchandise, / had already reduced their Halloween inventory), and surprisingly they had already Christmas merchandise.

You can use the first phrase if you mean "the shelves in the Halloween section were getting empty" and the second if you mean "space formerly allocated to Halloween supplies was re-allocated to something else"