I believe that slips are to worn shorter than the hem line of the outer garment.
At least I think that that is the prim and proper thing to do.
With the fashionable trend being short - long slips are as you noted found on ebay
or hidden in a closet lingerie drawer and not suitable for wear with the current fashion.
Also the trend is for women who used to wear slips to now wear jeans/pants/slacks as such.
Slips are not compatible with pants unless worn as pant liners but even that is stretching it.
So slips have become passé so to speak.
Slips do however make the wearing of a dress or skirt a great deal easier
keeping things in check wrinkle free and improving the smooth lines of the outer garment.
Another side plus is with woolen garments the slip shields the wearer from that itchiness factor.
But skirts and dresses are not really everyday wear these days.