Was it shipped in the factory packaging or was it the seller's packaging?
If the former it mitigates the seller's negligence but in either case he is still responsible to file a claim with the shipper and if he won't do that then he is absolutely failing you.
If he won't make it right or even lift a finger to make it right, then you should say that honestly in the feedback - "the sights were broken in shipping and the seller isn't lifting a finger to file a claim with the shipper, which is his duty." You only have something like 240 characters to say it so choose the words efficiently.
Don't even mention the packaging, that just introduces a point he can argue about. Sights were broken in shipping, seller has to file the claim, not the buyer, seller won't do it. End of story.
Whatever feedback you give, expect to get something equal in return. If that is the factory packaging expect to get bad feedback that the buyer is blaming the seller for something not under his control. The gunbroker feedback system sucks but so do most feedback systems - that's due to the nature of limited communication before a public audience. Basically it holds each person hostage to the other for poor feedback and whoever posts it first loses.
Sounds to me like you're not going to get new sights through any channels gunbroker offers, so here are two possible courses of action.
1. Buy new sights, forgive him and accept any karmic reward the universe offers you
2. Buy new sights and dox him here. Publish his business (not personal) name, address and phone number. There are plenty of folks here who would get a big kick out of letting him know they'll never deal with him due to the poor way he's treated you and at least as many who would harass him just for the fun of harassing someone. Let karma figure out what it wants to do.
