Do you have to have authorization from a company to sell a product you own that was made by them on ebay?

I have bought a whole bunch of supplements really cheap and I am now listing them on ebay to sell for a bit more then what I paid for. I just got a email from some guy claiming to represent these companies even though he never stated who he worked for and what companies he is talking about, and claims I need to end my auctions (even though he didn't say which ones to end as I have over 100 things listed from over 45+ companies) since I am not a authorized dealer for these products. It is a ebay auction and I bought and paid for these items (meaning I own them) so why would I need to be authorized to resale? If this is true how can ebay continue to function? Also if this was legit (and not my ebay competition) why would he not state his company's name, the companies who hired his company, or great me by my name. The email started off with a simple Hello before he called my listing unprofessional and continued to insult me. Also I got this email at 9:15 PM on Sunday. What company writes emails like this on a Sunday night? His ebay account he wrote me from was also new and just opened that day. Wouldn't a legit company who works for all these companies already have a ebay account? So I am thinking it is my competition, but to make sure, if it's not, what can I do and does this so called company have a leg to stand on if they took me to court?