Rare is a Meaningless Word
What the heck is that? Gotta be rare, right? Well, yes, in fact this brass main gear for an early American Flyer automatic reversing unit is quite difficult to find. But that doesn't make it expensive or even desirable, especially to someone who doesn't need one.
Conduct an eBay search using just the word 'rare' and you'll get tens of thousands of hits. Check Facebook Marketplace listings for your favorite Boomer Hobby and you'll see numerous headings proclaiming 'rare' and even 'ultra rare' whatever that's supposed to mean. The fact is, using and misusing this tediously overused word isn't going to help you sell anything because so many sellers are already doing exactly that to no avail.
Instead, follow the lead of successful advertising writers and focus on the unique selling proposition. What makes your item special? It might be an unusual color variation or even something as simple as the price you're willing to accept. Try free shipping for small lightweight items. Just don't tell everyone that whatever you have is rare, especially when it isn't - which is true 99% of the time.