2nd hand goods of any kind generally have poor resale values. Cars typically halve in value every 3 years which ties in broadly with your asking price. I'm sure if you try to sell some 6 year old sports kit (golf clubs, bicycles...) you'd be looking at a similar % return. Some items fall behind technically, all just get 'old'.

Some brands / ranges do appear to retain more value (Sonus Faber Homage series for one) over time but I think your experience is far from unique. Good luck with trying to shift it though.

Cinema: Meridian 271, 3 x DSP640 (behind screen), 4 x DSP320 (in-ceiling), 4 x DSP3100 (surrounds), 2 x AC12 (interface). 2 x Perlisten D12s subs, Lyngdorf MP40 processor. DT 145" 2.35 AT screen. Sony VW870 projector & S-T lens, Lumagen Radiance Pro.
Lounge: Roon, NUC, Kef LS50 Wireless + 2 x REL Gibraltar-2 subs

Meridian owner since 1998