Most non- specialist and non-safety critical trades. I’d never pay for gardening, landscaping, non gas safe plumbing, decoration, minor carpentry or minor construction. I can do it myself for less money, less hassle, and less waiting around.
any cleaning product that claims to be for a specific purpose- usually bollocks with the exception of acids for limescale and bleach for bacteria-ridden nooks like plug holes. Otherwise they are made of generic surfactants and disinfectants that you can buy much more cheaply as unbranded goods.
any beauty product without actually proven ingredients like super strong AHAs or retinol
any vitamins, minerals or supplements aside from for my vegetarian fussy eater - never saw any difference in taking them
any branded electronic peripherals like batteries, chargers, connector cables, accessories
most branded new clothing or shoes, exceptions being things the kids buy for themselves with their own savings
haircare at hairdressers - no one has noticed any difference and I look alright
full price meals out, cinema tickets, etc
mostly I don’t mind and don’t feel I’ve lost out, actually.
the one area I find that buying cheap really is akin to buying twice is with DIY products. B&Q own brand is criminally bad, as is cheap paint, garden and DIY machinery, hand tools, etc.
I will also never use a main dealer for car servicing or repair again. Why would I be paying toward someone else’s shiny new car and that massive showroom when I drive a clapped out 14 year old car?!