It happens when I put the cycling shirts and shirts away for another winter and get out the long tights and warm jackets.
When I see kit I haven't used for a year or two I think, "will I ever use that again"? If the answer is no, it goes on eBay.
Sometimes it sells, sometimes it doesn't. There's always a charity shop. In the last two months I've sold 19 items. They've been a mix of old sports kit, bags, rucksacks and camera equipment. I have substantially decluttered my kit boxes. And on eBay I've made a few quid.
The trick with eBay is knowing what price will satisfy you. If there's something that you'll never, ever use and you don't mind a relatively low price, then a 7-day auction is the way to go. I time them to finish about Friday 4pm.
You need to set a low initial price to attract buyers. But the bidding always happens in the last few minutes. Only one person might bid. So they might get a bargain. You might only receive that low, initial price. You have to be happy with that.
If it's something you feel is still really good and you won't be happy with a low bid, then you should use Buy It Now. If it's in good condition, set it at under half-retail price. If it's like new, then see what other sellers are doing and undercut them.
Then be patient.
I've sold a £600 video camera on eBay but it took a few months. As I write, I have a small drone (brand new) and a hard-shell Sony waterproof camcorder housing up for sale. They might be gone by the time you read this. The housing has been for sale for ages, and I've gradually reduced the price. I might do the same soon with the drone.
But best of all, I have decluttered.