Because they are in the business of selling guns--sometimes, particularly, uncommon or rare ones. Caveat emptor is implied in any transaction involving a Luger--they are under no obligation to do your homework.
A casual glance at Still's Imperial Lugers pp.62-63, 79-80 will reveal just how wrong this gun really is. This, along with the absurdly low asking price, should be enough to warn off any reasonably alert buyer.
(Note, I do not necessarily agree with the 'business is business' stance presented here, but it is the answer to the direct question.)
--Dwight
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