I've never bid on a $20,000 lot. But these situations (large qty of cheap bullion from a zero feedback seller) happen occasionally and I've placed a token bid on a few smaller lots. The last one I did, the auction got canceled early and then I started receiving warning messages from ebay security not to contact the seller. It sounded very serious. Be careful because the Buyers Protection only saves you from losing your money on the purchase. If the seller is running a scam, he will be receiving your name, address, and whatever other information you provide him/her (he'll also know you have a lot of money and probably a stack of bullion).
That's an excellent point. It's not quite as scary as Craigslist, where scammers can selectively target people who are geographically close enough for a "personal visit", but it still pays to be cautious. Using a PO box and a partial name or DBA on your eBay account can reduce your exposure.