So the only winners are stealers
Do you mean dealers, ie: bike retailers? If so then no, they sell their wares around the MRRP in order to continue trading, and the only people who pay for them are the ones that feel they are getting good value for money, which I presume is how you must have felt a month ago when you parted with £450.
The used market is not down to the retailers either, it's simply down to how much people are prepared to pay for something, and sadly with bikes it's not usually very much IME. The higher up the price scale you go the better it gets, but not immeasurably, and it can still be a right good kick in the goolies never the less.
Just curious, and you don't have to divulge, but why are you selling it so soon after purchase?
FYI and IME almost nothing depreciates like a bike, so don't think people are being harsh with you, they're just telling it how it is.
That's good. See if
Decathlon will give you something in writing to prove that to a potential buyer, it could help your negotiations/advertising.