IF you try on a sweater in a department store dressing room, but choose not to buy it, a persistent sales clerk won’t pursue you into the street yelling, “Hey, are you sure?” Nor will you receive a call at your home the next day to check again if you want to complete the purchase.

But in the online world, visitors to Web stores who touch the goods but leave without buying may be subjected instantaneously to “remarketing,” in the form of nagging e-mail messages or phone calls.

A new Web service, called Abandonment Tracker Pro, is in beta testing and scheduled for formal release next month. Developed by SeeWhy in Andover, Mass., the service will alert a subscribing Web store when a visitor places an item in a shopping cart or begins an application and does not complete the final step.

What distinguishes Abandonment Tracker Pro from other services is its enabling of remarketing “in real time,” SeeWhy says.