I'm not sure what an e-Bay listing smells like, but I don't think use of an auction to sell pieces is unique to e-Bay. Auctions have been around for a long time.
'Course it'd be kinda tough to hold an auction in the Marketplace as its limited to a single message per thread. If the seller does it right, they'll probably learn quickly that running an auction via e-mail is a lotta work.
There are several 'professional' secondary market dealers that use the Marketplace. And I believe there are a few regular HL dealers who have employees or others selling pieces at a discount for them through the Marketplace. They gotta love it cause its free.
Paul is the Marketmeister - he usually posts a message outlining the rules for that forum every couple of months or so. You might want to check it out.
Rgds,
__
/im