"More than anything search can do to clean up and improve the Marketplace experience, simply having a mechanism that discourages flood sales of the same item would do it."
Multiple listings of essentially the same product is against the TOS and is reportable by users of the SLM, though you are correct in that it doesn't stop people from doing it, and shoppers seldom report it.
An I totally agree that the same item in dozens of different textures, while not technically against the TOS, should be disallowed.
Personally I am against listing fees, LL already take their share. But I do see your point...