The event-only Auction House (in the Market) sells items for GEMS across three categories (winning-bid examples are server-specific reference only). Mysterious Goods (all servers compete, player-set bids): civilization scroll fragment x30 sets 4.2M-20M gems; Saint Michael Castle 20.4-22M; Famous City Key (Germany) 35-45M. Rare Goods (few servers compete, no buyout): Epic Historic General (Premium) x1 ~150-200k (lowest 53,000); 5K VIP Points x5 lowest 50,000; Senior March Size Increase x1 lowest 55,000; Dragon Scroll Chest x2 lowest 60,000; Achaemenidae Scroll Chest x2 lowest 60,000; Soul Crystal x30 lowest 52,000; 3-Day Speed Up x20 lowest 53,000. Common Goods (auto bid, Buy It Now buyout): 5M resource box x4 = 9,000/16,000; 8h Speed Up x30 = 11,000/51,000; 7-Day Speed Up x3 = 14,600/66,000; 100 Stamina x10 = 16,500/60,000; 1M Gold x8 = 14,600/64,000. Mechanics: Rare/Mysterious let you set bids; Common auto-priced with buyout. A bid in the last 5 minutes resets the deadline to 5 minutes. Gems paid at bid; refunded if you lose but must be manually claimed. For a mostly-F2P bot, the Auction is generally not worth gem spend except opportunistic Common buyouts; note the manual refund-claim step so gems are not lost.
Original research source: Evony Guide Wiki