Am I allowed to still sell on Amazon?

I received an email saying that I have been selling "counterfeit" iPhone cases. I buy from the same manufacturer and sell them as Used Like New. Am I in trouble and can they really suspend my account with no proof?

here is the email I was sent -

"Hello,

Just write to let you know that ESR case is the brand owner of ESR®. And we have applied a trademark registration in US already. Please stop sales on any counterfeit product immediately, otherwise your selling privileges will be suspended or terminated according to Amazon sales policy. And we will reserve the rights to investigate and affix your legal liability globally. The ASIN is: B0743577RN
We will claim it to Amazon if you go on selling counterfeit product and no reply in 24hrs.

That is a very poorly written email; so that would be my first concern about it being ffake

Did the email have your name on it or just hello? If it did not have your name on it, then that is truly a fake email. How could they address Mr. Hello

Second, if you are being investigated for selling counterfeit, you would get a certified letter from their attorney, not an email, and you would receive a correspondence from Amazon and probably a notice from US customs who investigate the sales of counterfeit merchandise and the US postal inspectors who investigate postal frauds

Whether you choose to sell it as new or used, it doesn't matter. It is unethical and probably illegal to represent it as something it is not. You can't use someone else's trademark to sell your product.

Consider your source for stock. You stock from the OEM original source(genuine) you are clear of any wrong doing.
You stock from cheap Chinese knockoff, you violate trade laws on top of everything else unless you specify your items are not the genuine item.
You may not even be aware if you are the middle man, buying from non reputable wholesalers marketing cheap clones under guise as genuine.
Nike is the most cloned name brand at the moment, 90% of the time, you are buying cheap Chinese, NOT Nike.
Use care in business as a marketer.
Another point, is consider your source of your e-mail, Competitors may send out such e-mails spoofing sales outlets like Amazon or E-bay. Check the validity of your e-mail.
A dog eat dog world.