You could also buy the second to top D in giran stores and upgrade it yourself at blacksmith mammon, ends up slightly cheaper than buying from ppl who sell the elven long sword on the market. but it takes ancient adena to do so also
In my opinion the top D weapon is not worth the money for the last 2 or 3 levels you could need it.
The first C grade weapon is cheaper than the market price of an Elven Longsword.
u could always look in the library for information of weapons for certain grades, but it hasnt been updated for 2 updates. or you could go to www.l2guru.com/l2clac (http://www.l2guru.com/l2calc) for detailed information of weapons and gear.