Yes, as long as there is no pressure placed on colleagues to make a purchase and it interferes with neither your working hours nor the work facilities made available by CERN.

The Staff Rules and Regulations (R I 13.04) require the prior written authorisation of the Director-General for members of personnel making any communication outside the Organisation in which they...

You should seek prior authorization from the Director-General before agreeing to give the lecture. Even if you receive authorization, you may not derive personal benefit from the activity because it...

Employed members of personnel must obtain the prior authorization of the Director-General before accepting a teaching position.

Yes, provided that you participate without your connection with CERN being identifiable.

Discuss the situation with your hierarchy.  You should not work on any task that could create a conflict of interest, or the appearance thereof, between CERN and your future employer.
Bear in mind that...