Two issues
Amount of Material Goods Due to Cheap Mass production
I think part of the problem is people have too much stuff because there is a lot of mass produced crap from China/ Far East that is very cheap - especially clothes, electronics and plastic stuff. thirty years ago a plain white t-shirt would be more expensive, good quality and last ages. people looked after them better.
Now you can get a shitty white -shirt for £2 that will look ok for a few wears, maybe one wash and then be crap. So people buy another one but don't chuck out the last one thinking it will do for home lazing. Shoot forward 10 years = approximately 200 shitty white t-shirts that need disposing of. Everyone is trying to do the same so you have no chance of selling anything that isn't basically unworn or new or designer. Because if a person wants one, they too can buy a new one for next to nothing.
Cost/time benefit of selling
If you think of your time as having a cost, it doesn't even have to be your actual wage. Minimum wage is about £11.40 or so an hour.
Think of selling something and time expended. You have to photograph it nicely (time to photograph, arrange it, select photographs). List it (time taken to write what it is, set a price, upload photos). Probably spend time keeping a watch on the selling site (checking in or checking emails). Responding to queries from buyers. Pack it up. Go to post office/ drop off point to dispatch it.
That all adds up to a lot of time in most cases it will be well over half an hour and probably closer to an hour including travel, queueing time for dispatch. By the time you have discounted the postage costs,packaging costs, possible petrol to drop off point unless you are selling something for over £11.40 you have wasted your time really because you've made so little v time expended. You could have had more bang for your buck doing something nice for yourself like having a bath or playing with your children or watching your favorite show.
I think more and more people realise this - that selling tat is a fools errand and makes you no profit and is a waste of time - so end up giving stuff away for free.