While hanging out in the Splinterlands Discord trying to create a card owner rental whitelist (people who won't cancel during end of season), a game veteran chimed in:
A couple of hours earlier, he had just completed one of the biggest single card purchase in Splinterlands.
It was for a very rare Level 4 Golden Prince Julian Dragon Summoner card:
The Prince Julian card was a Mystery Potion Reward Promo Card from the early days of Splinterlands.
See the transaction here:
https://hiveblocks.com/tx/0782c95bb730fe576a730da19d4fe1fea0a195cf
Since 
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