This is a very good question. The bottom line: Any products which you legally purchased can be resold without fear of copyright infringement. There was a recent Supreme Court case that even held that applied to so-called grey market goods (goods purchased overseas where they are cheaper).
That would suggest that any products that you properly purchased are not infringing goods. So the question becomes did the "reputable online retailer" have the right to sell you the return items or were they really supposed to go back to the manufacturers. I don't think there is any way you could know that, but it is certainly reasonable to rely on the representations of the retailer who says it can.
My guess is you got the takedown notice because there is no easy way to tell whether you are selling properly licensed products or just knock-offs. Be careful that you didn't really buy knock-offs rather than legitimate products.
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If you are talking about "Insanity"......
VeRO has a problem with your listing this item, so sell it on Craig's list, in a local garage sale, etc. Do Not relist on eBay.
The company has a blanket prohibit because it has lost tens of millions on rip off sales via eBay.
Because (1) they may not be genuine or (2) when the DVD's were originally "sold" they may have only been licensed, not sold, to the retailer and that license may prohibit any further re-sales. Only your own attorney can review the pedigree of the products you purchased to determine if you can lawfully re-sell them. Do not send eBay a DMCA "counter notice" in response to the take-down without first speaking with your own intellectual property attorney.
The above response is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.
"Insanity" right? My understanding is that they are not sold, but licensed, so your "returns" are likely not legitimate to resell. See a copyright lawyer in CA to make the precise call on that. If my understanding is correct, you bought a pig in a poke and are stuck unless you dump them illegally as one colleague recommends. I do not recommend knowingly illegal actions by Askers. And, since you are in Pasadena, you are in the home turf of their lawyer and likely to get nailed. I think the better course might be to eat the DVDs and avoid the risk, which is substantial, rather than risk a sale, UNLESS you see a copyright lawyer. Don't think that because others get away with it you will, too. YOU are on their radar, likely due to your large sales volume. Be careful and get that attorney. There are lots of good copyright attorneys in the LA area. Use the Find A Lawyer tab and consult with one. I would not suggest ignoring this CDL.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.