Is it fair to tell another buyer how much we paid for an item?
You don't have to tell them how much you paid for it originally.
They can do their own research on the internet if they want.
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They only have to look at the listing and bidding history to see what you paid.
As said above, the price is clearly visible if anyone checks the bidding history. I presume it's the x bike which shows as £400 accepted.
The seller has listed another one, so the other member probably want to know what sort of offer they should make.