The reason they do that is because of the way eBay fees are charged. You pay a certain amount to list the item, then at the end of auction, you pay a final value fee I think its called.
Basically, the more money that the item is sold for, the higher the final value fee. For an item between $50-1000 it's 12% of $50 + 6% of the remainder. So if an item sells for $600, that's almost $40 in fees.
Of course, doing that to get around the fees is against eBay's rules. If someone reports it, the listing will get removed.