Can a company restrict what you do with their item after the sale?

Why I am asking this is because I was selling some items on eBay and received email notice that the original company is stating that these items are now fakes or counterfeits. eBay did not pull my auctions so I do not know if this company contacted eBay or if eBay chose not to do anything. These items are not fakes. I have receipts from that very company or one of their own representatives. It is almost like this company is saying that once they sell it to you, it becomes a fake once it leaves their door. Anyway, this company is threatening me by saying it will take me to court because of this. I really don't know how serious this company is about taking a small time seller to court. I only had 5 listings per week. Right now, there are several sellers that are selling the exact same items on eBay with a lot more listings. And, no they are not company representatives. I know I can prove that these items are what they are. Yet, is it worth going to court to protect my rights? I know about the Lanham Act, First Sale and Fair Use Doctrines. Has anyone really won a case based on these doctrines?