When you say custom bike, do you mean the frame was custom built for you, or just you’ve picked the components you wanted to fit to a stock frame?
The real answer is to split and stick everything on eBay with a 99p starting price and no reserve!
My feeling is once you start getting above a certain price point people aren’t looking for a complete bike. Not sure I would drop anything over £1k on a secondhand bike (though ironically I might if I’d bought all the bits seperately, go figure…).
People are either upgrading from, say XT to XTR, or a frame to retrofit all their current nice kit. Custom specs on eBay always seem to be the ones asking top dollar.