nono we all learned this so can you bro! all you have to do is open a second browser window using the keys (ctrl n) goto the posting then highlight the web address in the header bar and copy that by hitting the keys (ctrl c) then goto to your forum posting type what you want to say then (ctrl p) the link will then appear in the post.
Or you could just write it down from the graigs list post (probably summthin like http://graiglist/sfbay/forsale/trailer.....etc) and then manually type that in your post good luck!
if your using a mac then subctrl for command key