Trying to post feedback-URL "invalid" HELP

[Rumblemonkey]

So I tried to complete a purchase from an old timer, (Cartridge Calls) but he flaked out on completing the deal. I tried to leave negative feedback, but all the URL's come up as "invalid. I tried the PM string URL, the forum category URL, and the actual add URL, but all come up as invalid. What do I need to do to correctly fill in the URL requirement? Thanks, sorry this transaction did not work out! See my PM's for the string-I contacted him 8 times hoping to complete the deal!