Many big Amazon sellers received a notice this week from a company they rely on for international money transfers. The message from WorldFirst, which is based in London, was alarming: its U.S. business was closing immediately.

"We are writing to share some news that affects you as a US-based customer of WorldFirst," the email said. "The WorldFirst shareholders have taken the decision to discontinue with the US operations. As such, we will no longer be able to offer our products and services to you."

WorldFirst didn't offer an explanation for the abrupt decision, and one Amazon seller, who asked not to be named, told CNBC "that was quite a shock." The company said its U.S. business was being rebranded as Omega, which would operate independently from WorldFirst, and that no new transactions could be made after Wednesday, Jan. 30.

Sellers and competitors who spoke with CNBC speculated that the shutdown was directly related to a pending acquisition of the parent company by Chinese fin-tech giant Ant Financial.

Ant, an Alibaba affiliate and parent of Alipay, has reportedly been in advanced talks since late December to buy WorldFirst for about $700 million. Ant has been unable to crack the U.S. market because of opposition from the Trump administration. Last year, the Committee on Foreign Investment in the United States (CFIUS) quashed Ant's attempt to buy MoneyGram for $1.2 billion because of national security concerns.