It's impossible to know what Chanel will do. Trademark infringement, however, is a serious matter, and many companies zealously protect their mark. You may wish to take the earrings off your site, even if they were not infringing on Chanel's mark, so as to reduce the risk of litigation.
If you are seriously concerned, you should contact a lawyer experienced in infringement matters to at least write a letter to Chanel to explain and defuse the situation. There are many qualified lawyers on AVVO who can help you with this.
So, what is missing here is the problem. It is not illegal for you to sell a pair of actual Chanel earrings on the internet.
Can we all assume then that these are not the real branded product but knock-offs?
How you handle this really depends. We have several such cases on our desk at present and each present very different scenarios and are being handled in different ways.
I suggest you discuss everything over with a lawyer in private (and be completely honest with the lawyer regards to how many you sold, etc.). Then a best course of action can be reached. Most of us here, including myself, offer a free phone consult.
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For one pair of earings it is unlikely, but it could happen if the attorneys for Chanel percieive that you have assets and/or want to make an example of you.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
Anyone in the chain of commerce of trademark infringing goods is liable to the actual rightsholder. That includes you, since you probably used the Chanel TM in your ad for the listing.
It doesn't matter that you only infringed a little with 1 listing, or that the sale had already been completed. Lots of Avvo posters use the opposite argument, that if their online listing was removed, they shouldn't be liable because they were caught before the sale could consummate.
Here you may have sold the counterfeit earrings to the rightsholder's lawyer's investigator who wanted proof that these earrings were counterfeit.
You probably won't be sued, but you may very well receive a demand letter demanding a monetary settlement and your agreement not to traffic in counterfeits of their goods again (if that's not what's in the email you've already gotten).
Trademark owner are REQUIRED to police their TMs or they're in danger of losing their rights. So that's why they're sometimes aggressive with counterfeiters.
Avvo doesn't pay us for these responses, and I'm not your lawyer just because I answer this question or respond to any follow-up comments. If you want to hire me, please contact me. Otherwise, please don't expect a further response. We need an actual written agreement to form an attorney-client relationship. I'm only licensed in CA and you shouldn't rely on this answer, since each state has different laws, each situation is fact specific, and it's impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue.