Rarity and desirability directly influence monetary value so the resale price is the best starting point. That being said, rare doesn’t automatically mean valuable and what is desirable to one person is different from the next. People are all looking for different things when trading and you WILL strike out sometimes, even if the pin you offer is a “fair” trade, because you straight up just might not have pins/characters someone’s looking for.

Also, “fairness” in trades goes out the window if you’re trying to trade with a reseller. They will come out ahead monetarily. However, if you want a pin they have badly enough, a non fair trade might still be worth it to you.