Seller clearly acknowledged sights were damaged in shipping, so it's 100% on him.
Get Gunbroker involved. Find out price of equivalent sights and ask Gunbroker to 1) refund you the amount replacement sights would cost, 2) refund your the price of the sight (and subsequently reduce the amount paid to seller) or 3) have Gunbroker cover return shipping (which reputable sellers do as part of their contract with GB). Although not GB, I had similar with crossbow purchase on eBay and got them involved; matter resolved in week and eBay actually refunded me $169 (of a $450 purchase) because seller acknowledged issue with scope (similar to your guy admitting poor packaging).
Unethical sellers need to get stung HARD financially, else they continue their crappy practices and give GB a bad name.
Full disclosure: I've bought several firearms using Gunbroker and have had zero issues with sellers or items received.
