The person who paid $500 resold it for 200 Keys.

https://backpack.tf/profiles/76561198088253565#!/compare/1600387200/1600473600 - Seller gets a DP Polar

https://backpack.tf/profiles/76561198161711912#!/compare/1600387200/1600473600 - Buyer loses 210 Keys (200 for the Polar)

https://backpack.tf/profiles/76561198163116031#!/compare/1600387200/1600473600 - Polar seller gets 200 (185 show, 15 others used up)

https://imgur.com/a/y6gx1h7 - Polar buyout of 200 Keys

Some of the 210 Keys went to Darrell (the Polar seller) and were sold immediately to quicksell.store, hence why only 185 show.

Additionally, the lone key in Duck's (the Space seller) backpack came from Yoink (the Space buyer), showing it went Space -> Keys -> Polar.

I would go 166 - 200 Keys here.