How to report with apple that there is a fake apple seller who’s doing a spam

Contacting support won't help in this matter. This is the specific answer to your "How to report..." question:

Apple - Legal - Counterfeit Questions

That is a form to report intellectual property infringement issues, such as the one you found the one you found. Goes straight to Apple lawyers

shreya26 Said:

“I came across a page on instagram who’s a scam and using Apple dublicate products to earn money. How to report against them”

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Contact Apple Over-the-Phone:

Contact Apple over-the-phone, and perform a screensharing session with them. That way they can see this, firsthand.

Contacting Apple Support:

• Contact Us - Choose Locations
• Proceed from there as necessary

Note: Phone calls are taking a bit to go through at the moment, due do lots of calls. With the Coronavirus Pandemic, many are at home at the moment, with much time on their hands to contact Apple. So, just stay on the line, and you will get thorough :)

Allan Jones Said:

"Contacting support won't help in this matter.[...]"

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How it will help - Apple's Reps will see your screen and will know that is going on here. Just talking about it via voice (BWOM), and all you can do is describe it, verbally. So show this to them - they will know what, if anything, is needed to be investigated. I can't emphasize it enough to the OP.

Use the link Allan Jones provided and include the URL (web address) of the site where you found it. Support is not the right department for legal or copyright issues and they are extraordinarily busy right now, so there is no reason to bother them with things they will have to send to the right department anyway. And, never offer to screenshare unless you know who it is you are talking to.

You can emphasize it all you want but they do not need to "see" this on the OPs screen and this just would add to the overload of support needlessly. The OP can use the correct contact point, the one in Allan Jones post, and merely give them the name of the Instagram site. Then Apple's legal people can look for themselves. No different than giving someone a URL.

Contact Support