Hi everybody
The plural of serie is series.
However, we talk about "(una ) serie de acontecimientos, several, a series of happenings.
The singular form of the word is used because we are referring to ONE group or set of things.
(Rae or other dictionaries do not show the definition of the plural form of
any word, unless it has a different meaning)
With products , on the other hand, I would go with
gentle's input: (una ) línea de productos
línea profesional, línea de productos profesionales, línea de productos de uso profesional.