IMO and my best guess is the the spoiler market price was set prior to the spoiler being available on shop.tesla.com (it became available in early to mid 2020...I don't really remember exactly when). Before it was on tesla's site, I used to look at the them whenever they popped up in the for sale forum or eBay and the price was generally around $300 for new unused/never attached and around $250 for removed/adhesive needs to be removed/adhesive needs to be reapplied. Some people have tried to sell them in the for sale forum for 800 since that's what they are going for on shop.tesla.com but it doesn't seem like they ever move at that price. From what I've seen, they drop the price to around $300 and then it gets sold.
I bought an used oem spoiler on this forum for around $200 (Originally asked $700). I had to remove and reapply the adhesive...it wasn't too bad, it just takes a lot longer than you think it will. And 1 of the ends of the spoiler pulled off after a few weeks...I've been too lazy to try to make it better.
Good luck!