This is something I have to regularly remind myself of when I'm having trouble with 'I might need this someday' or 'This was really expensive I can't just get rid of it', when trying to declutter.
Things that have been bought in the past will hardly ever bring you more money when you get rid of them, obviously there are exceptions with vintage items. Day to day items will become outdated, your taste may change, you don't use them anymore, or they will just look a bit tired.
I have sold a few items sucessfully such as my accessories for my Kenwood Chef but selling is time consuming with taking photos, answering questions, packaging the item and dispatching. If you worked out the time spent, even just paying yourself minimum wage, it is hardly ever worth it.
The money has been spent. If you have thought carefully before your purchase its likely your money hasn't been 'wasted' the item served a purpose at the time. The thought of the money being wasted, the sunk cost fallacy, causes us to over-inflate the value of an item which in turn makes us keep hold of it.
Getting rid of an item doesn't make you have less money today. Loss aversion is a very powerful experience and it is hard to let go of the thought of money or time that has been invested in the past. It can also make us spend more money by trying to upgrade or mend the item. 'Don't throw good money after bad' which apparently was a saying originating in the 18th Century!
Don't think of the money as lost, think about the time, energy and space gained through not having it. Once you start thinking this way it becomes easier, in a way you are paying for a new intentional, simplified life.