Here's a quick guide explaining the differences between the two styles of knives.
Both automatic and spring assisted knives contain either a spring or torsion bar that engages to open the blade. Both can be easily opened with 1 hand. Automatic knives are not permitted on Ebay as they are illegal under federal law. Spring assisted knives are permitted on Ebay and throughout the US (unless prohibited for other reasons, e.g. blade length). Although they often look similar, there are slight but significant differences.
The primary difference is the method of engaging the spring.
Spring assisted knives require the user to manually begin to open the blade (via a thumb stud, finger nick or notch, or pressing a portion of the blade that protrudes from the back of the handle..called an index) before the spring engages. Typically, these knives require the user to open the blade approximately 30% before the spring takes over fully extends the blade.
Automatic knives, however, use a push button or similar "trigger" mechanism to initiate the spring. With an automatic knife, the blade goes from an entirely closed position to a fully open position via the force of the spring/torsion bar alone. It is this fact that determines whether they are legally considered an automatic opening knife. These are often referred to as switchblades or push button knives.
Another style of automatic knife is the otf or out-the-front knife. These open via a "trigger" as other automatic knives, but the blade protrudes from the front of the handle housing.
Any style of automatic knife is prohibited on Ebay and should be reported! A seller who is not honest with Ebay shouldn't be trusted by anyone.
Spring assisted knives are often used by rescue personnel and others who need the capability of quickly opening a knife with 1 hand. The added capability that the assisted opening offers makes these a more useful tool. With the wide variety of assisted opening knives available, and the similar capabilities as automatic knives, there is no need for a person to possess a knife that is both illegal and Ebay prohibited.
If you're looking for a great knife/tool, try an assisted opening knife; stay legal and safe!