Teresa Sunna, 28, took the additive as part of a food intolerance test in a private clinic in the southern Italian town of Barletta. Two other patients who underwent the same test are recovering in hospital.
Italian authorities have launched an investigation and police in the northern town of Padua seized 1,000 tonnes of sorbitol from local businesses over the weekend.
The Italian Healthy Ministry released a statement on Sunday advising anyone who had purchased sorbitol on eBay to not use it and to contact the police.
EBay said it was "saddened" by Sunna's death in a statement and said it was cooperating with authorities.
Sorbitol is used to sweeten products such as cakes, ice cream and diet foods and is widely available to buy legally online. EBay, which previously carried sorbitol along with many other chemical products, said it was blocking sales as a precautionary measure until the cause of death was clarified.
(Reporting by Catherine Hornby; Editing by David Holmes)