I've done similar with online groups like freecycle when selling or giving away things. Include "to prove you've actually read this post, mention the word elephant in your response" in the advert and have my mail agent delete everything to that address[1] which doesn't include the word.

Massively reduces the number of messages you get from time wasters who just mash "can I have this?" or "is this still available?" to get in first, who will then ghost you because they've taken an extra half second to decide they don't want the item after all[2] so you spend time writing replies that achieve nothing.

Probably means some genuinely interested people get redirected to trash too, but if the item is worth having there will be enough responders to make this not matter. If you see no responses after a time, maybe then check your trash to see if there are any there worth responding to.

[1] those services attract much spam, always have a specific address for each that you can turn off once the junk starts flowing in

[2] as it isn't the right colour, or the right size, or you can't deliver, or will deliver but can't conveniently offer collection, or they completely misread the subject line, ...