How to Determine the Value of a Used Pen

How used is a pen that's still available brand new on the market? If it's heavily used, it's probably like half off. I don't have a hard and fast rule, but I feel like if a product's available, you've bought it and used it, it's no more than half for me. One of the good places to go to sell pens is Reddit, pen swap or slash r, pen underscore swap. The Panatic Slack is always has a good buy sell trade,. Like it's a community driven tool where you're not there to just sell pens.