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DavidIt depends on who is selling. For the Sotheby's/Ebay auctions, the seller is sometimes Sotheby's but usually is one of a variety of independent auction house/dealer affiliates working under the Sotheby's umbrella. Sotheby's 'in house' auctions are under the eBay name 'Sothebys.com' or similar.
From the limited stuff I've looked at (sport and nonsport), the prices are usually high, on occasion comically. However, I got a deal on one lot from one of the sellers, and was satisfied with the product.