Where can you find buyers for 8-track music cartridges?

Where can you find buyers for 8-track music cartridges?
You could try audiophile communities online.

There are very few people dedicated to 8 track collecting. Vintage music collectors prefer records because they do not degrade and are easier to maintain. 8 tracks on the other hand eventually disintegrate and are not considered a long term storage medium for quality sound.

The only people who buy them are generally interested in them as historical pieces and tend towards collecting a wider range of music and sound equipment, hence audiophile communities and those who collect music and radio equipment, not just records.

Beyond that most people tend to end up with 8 track collections more by accident, handed down from a relative. You will have a hard time finding buyers; you can find tons of old 8 tracks being sold at libraries and pawn shops for 25 to 50 cents, and they rarely sell.