Not really... at least not a way that a Gunbroker buyer can find your comments easily. Contacting customer service... stays with customer service, and they won't do anything at all unless a policy is blatantly abused. The Gunbroker "maze" can be difficult at best, and as you didn't buy, you have no access to feedback... other than to the GB forums, and a complaint there would get yanked pretty fast.
If you belong to any social networks where you have your very own page and can say anything you want without reprisal... I suppose that'd be a way to go, but someone would have to be searching for the seller outside of the GB search engine.
I know it's a bit raw right now... but I'd respectfully offer the suggestion that you consider this for awhile. I've been on both sides of that fence, and after a time, a cooler head tends to be smarter more thoughtful head... even with those we'd just as soon strangle at the time.