PSA: Wear the shoes you love, and sell the shoes you don't
First things first - any "universal" rule is obviously not going to apply to everybody and will always be a bit tongue in cheek. These are some observations and reflections based on my own purchasing and hoarding behaviour that I've seen mirrored in various online communities. If it helps to save you some of the cost and faff that I've had then all the better. If not then the worst-case scenario is that you have too many shoes, which is hardly the worst thing in the world.
They're too nice / too formal / too casual / too unusual to wear often
My pet peeve (when commuting used to be a thing) was a nicely polished pair of shoes that was scuffed on the tube by a stumbling commuter who thought they were too cool and immune to the laws of physics to need to hold onto the handrail. Similarly, I've had so many pairs that I would never risk wearing if the weather showed the remotest chance of turning inclement.
It's important here to remember that shoes are ultimately just objects - if the risks of using an object dissuade you from using it completely, there is little point in owning it.
Consider also shoes that are so niche in style that fit only fit one specific need - this largely depends on how much space you have, and how much it bothers you keeping shoes gathering dust that only get worn twice a decade. Things like patent leather oxfords are a contender here - they're great when you need them, but a pair of nice black wholecuts can fill the same niche but also be wearable on non-tuxedo occasions.
Overall, if they are too "anything" to wear regularly, consider ditching them.
They round out my collection
Look, I get this one. I frequently look at my collection in terms of what can "complete" it or round it out.
The sad reality of materialistic pursuits (and this is most certainly one of them) is that the collection will never be "done". The endorphin rush of a new pair will fade, and you will start eyeing up new options. This is exacerbated by engaging with social media content that is founded on the never-ending pursuit of "more".
In a rather more depressing sense, remember that there is a 99.99% chance that nobody except you really cares about your collection. I know that sounds harsh, and maybe you do have a lot of Instagram followers or Reddit posts that would imply otherwise, but the meat and bones of what a collection costs you and the value you get from it is a thing that concerns you and you alone. On that basis, spare yourself the pressure of pursuing a "perfect" collection - it does not exist.
They're a bargain!
Funnily enough, the more I write this piece, the more it feels like a personal attack on myself.
Psychologically the appeal of the deal is a very well-established phenomenon - but just remember, any unplanned expense is not money saved, it is still money spent. If you see a pair of shoes for £300 that would be £1000 new, you haven't saved £700 spending them - you've spent £300. The only person you are £700 better off than is the parallel version of yourself that bought them full price.
Not to say you shouldn't be looking for the best price possible - almost everything I buy is from eBay or charity shops - full price purchases are a rare beast indeed.
They almost fit
You know what things that "almost" fit don't do?
They don't fit.
The Edward Green pair pictured above were beautiful, but would have required some pretty aggressive width stretching to get right. I've written previously on this topic around some of the less and more forceful interventions you can make to modify fit, but would stand by the message from that article - if they don't fit, don't waste time and money trying to change that.
The more I've owned shoes, the more I've seen fit as a fairly binary situation. Shoes that are too big and need insoles; shoes that are too small and need thin socks or stretching... just don't worry about it. There are a million other pairs out there that will fit you without that. Do you really want to be factoring in which blister plasters you'll need in the evening when picking out your shoes in the morning?
I'm sure some will disagree on this point and say fit is a spectrum and too subjective to be put into such neat camps. For my part, it's clear when a pair does and doesn't fit.
I'll never find another pair like them
Another one that I massively sympathise with. I'm checking eBay constantly for interesting pairs, and I've seen plenty that were pretty unique chances. Really rare makeups or obscure makers, like the pictured pair of alligator Chelsea boots from Gaziano & Girling. RRP for these would be stratospheric, probably in the region of £8k (fully conscious that opinion is likely to be pretty split on these, but I think they are pretty cool!)
And the reality is that now, looking back on it... I can hardly remember any of the pairs I missed. This is the way the human brain works. These opportunities come and go. As we said earlier, the nature of the materialistic pursuit of newness is that these missed opportunities are shunted from the brain pretty quickly.