Not money "saving", but certainly money raising.... We sell virtually everything we don't use anymore on ebay or preloved. Especially bundles of low value individual items like books, DVDs, computer games, clothes, toys, etc.
Postage costs make selling most books unviable, but put 4 or 5 in a bundle with a single postage cost of £3.20 and you'll usually sell for a tenner, leaving you with a fiver or so after costs. Same with clothing, both adult or childrens, put on a bundle of 5 t shirts or 2 or 3 pairs of shorts etc and you'll clear a fiver or a tenner. My OH lost a lot of weight and we sold a load of his used work shirts, trousers, etc.
Same with DS's toys, as he grew older, we'd sell the prior year's birthday and christmas presents that he'd stopped using. We keep the boxes/instructions so you can usually get pretty good prices for games, construction toys (Lego, mecanno, electric kits etc) if they're boxed, complete and have instructions! He also had a phase of model railways and we sold most of that at a profit as prices rose, especially on some models which quickly became rare (such as Hornby Thomas the tank engine where we bought a particular engine for £70 and sold it for £250!, but only because it was pristine condition and we'd kept the box etc). Just sold a Lego robotics set for £300 that we'd bought for £250 about 5 years ago.
I've sold most of my older dresses, leather jacket, coats, etc., but again, kept in excellent condition, not much usage, stored properly (coat bags etc).
We also sell furniture, carpets, etc rather than putting them in a skip - lots of people want it when moving or kitting out at bed to let and some are even happy to take up a carpet or dismantle a wardrobe so we don't even have to do the "dirty" work ourselves.
By selling older stuff, we can "churn" the proceeds into newer stuff.
For the really cheap items, and things that have been well used, etc., we go to a car boot sale once a year - we've never come away with less than £100 after pitch fees and sometimes manage nearer £200. That's just selling loads of things for a pound or two each, such as a bundle of comics, or used unbranded clothes bundles, or even surplus rolls of wallpaper, partly used bottles of perfume, etc