EBay has filed a lawsuit against Amazon, accusing the US retail giant of using illegal tactics to recruit sellers.
It says Amazon representatives abused eBay's internal email system to contact sellers - a violation of the marketplace's policies.
Amazon declined to comment on the case, which follows a letter from eBay demanding an end to the activity.
It had previously said that it was investigating the claims.
In the lawsuit filed in Santa Clara County, California, eBay says Amazon representatives created eBay accounts to solicit sellers, often sending messages within minutes of setting up their profiles.
The activity dates back to at least 2015 and involved dozens of Amazon representatives, who each sent hundreds of emails, it says.
EBay was alerted to the issue a few weeks ago by a seller and asked Amazon to stop.
The lawsuit says the messages show Amazon was aware its practices violated eBay's use policies.
It says Amazon's practices "appear to be part of a larger pattern of aggressive, unscrupulous conduct" by the Seattle company.
EBay, which also owns the ticket resale site StubHub, is seeking monetary damages to be awarded at jury trial.
The California company, which launched in 1995, was an online shopping pioneer, developing a platform to connect buyers to sellers.
It now boasts more than 175 million active buyers, with sales of nearly $90bn via the platform last year,
But its lustre has faded amid increased competition, especially as Amazon's dominance lures more merchants to its own platform.
Last year, more than half of the items sold by Amazon came from third-party merchants.
Amazon overall is estimated to account for about half of retail dollars spent online in the US.