What are some specific products* that are evidence of the cosmetic industry and/or toiletry industry convincing us of their necessity yet in

All makeup products - eyebrow pencils, rouge, blush, eyeliner, mascara, lipstick, (etc.).  None of these items are "necessitates", however they have a high mark-up.

Perfumes.
A pleasant smell is neither useful nor necessary.
Often, other people find the fragrances offensive or even allergy triggering.
A clean, healthy body never needs perfumes,
and no perfume can mask BO, alcohol, illness, or tobacco breath.

This includes household and toilet "air-fresheners". 
A clean house doesn't stink.

This post was edited by inky at December 21, 2020 8:19 AM MST

Face toner.  I stopped paying $20 for a little bottle of Clinque toner when I was around 35.  When I learned that the ingredients in ingredient lists were listed according to their prominence and realized that it was mostly witch hazel, I started paying $.89 for a bottle of witch hazel rather than $20 for Clinique toner.   Very thrifty, don't you agree?