What to do with replaced tweeters

Hi - so, not naming any brands or companies here, but about a year ago I upgraded my speakers by installing new beryllium tweeters. I still have the old and undamaged tweeters. However, I've looked on ebay and on here and on other audiomarts and there's none of this particular, mostly well-known-among-audiophiles tweeters (or woofers, for that matter) on the market. So, where does one sell unwanted tweeters? And how does one determine a value to set? Should I just keep them "in case"?

Again, not trying to make a sale here; but trying (perhaps ignorantly) to figure out what to do.

Warm regards,

Simao