I can understand both of your positions. He was in the right when you sent the bag without the straps and the buckles. The strap keepers kind of sound ridiculous - I would have sent him $5 on PayPal and told him you were sorry you forgot them and to go buy some new ones. The condition of the bag is subjective, so that’s a push.
I’ve had similar experiences. I bought a bino harness on here and it showed up without the straps that attach to the binos. The seller couldn’t find them after I contacted him, so he paid $10 for new straps and I covered the $4 in shipping. Seemed fair that I wanted the required items for the harness and I felt like covering the $4 shipping wasn’t a deal breaker... I mean, it’s $4! I also sold some binoculars and when the buyer received them, he felt like I had misrepresented the condition in the ad and he had not seen in the ad the exact version of the binos (he thought they were HD when they clearly weren’t, from the pics and description). I offered to take them back, and he covered return shipping. When I got them back, I showed my wife my ad description and gave her the binos and let her judge. She voted for him, so I refunded all of his return shipping cost. There is no one right way to solve a problem like this, so I just use common sense and the golden rule, and I haven’t been banned yet!