In a statement to the Wall Street Journal, eBay called Amazon's behavior "unlawful and troubling" and hinted at the possibility of a lawsuit, saying it would "take the appropriate steps, as needed, to protect eBay." Amazon said it was investigating the claims.

It's not known if Amazon will fight the cease-and-desist request. However, there are high stakes for both sides. eBay is entirely reliant on third-party sellers, and could face serious trouble if enough merchants jumped ship. Amazon, meanwhile, makes more profit from third-party marketplace sales than its own thanks to both its 15 percent cut and fees. Whether or not the poaching claims are true, Amazon likely doesn't want anything slowing down its ability to attract sellers and improve its bottom line.