People see value in all sorts of things, but the value to them is not necessarily shared.
I try to go by a very loose rule, if I've not used something in about 5 years (and I can't see a definite future application for it), then I'm looking to get it out the door. Having said that, I have a sweatshirt that's 40 years old and T-shirts about 30. I also try to weigh up the cost of buying the thing again should I need it. Quite often, buying it again is a cheaper, better option than having lots of stuff lying around not being used.
The other thing is that new things come out so quickly that older, decent stuff ends up having no value but takes up space. Just seems wrong to bin it.
Answer, just try to give stuff away if nobody wants to buy it, and just hope they'll make some good use of it.