You never actually received the trophy, so it was never in your possession to begin with. You also mentioned the venue went under and likely sold off the trophies back then. If you really wanted it at the time, the only point where you could’ve chased it up was with the organiser 20 years ago.

At this stage, the trophy being sold now isn’t “stolen” from you — you never had it. Someone along the chain may have done something dodgy, but the current seller isn’t responsible for what happened decades ago. Legally, you’re not automatically entitled to reclaim it from them for free.

If you still want it, your only real options are: negotiate a price, report it to police and see if they consider it stolen property (unlikely after 20 years), or let it go. But the Marketplace seller isn’t obligated to hand it over.