I have two thoughts:
Firstly, if he’s sold you broken kit, what else isn’t right about it all. Having a few of those glocks, it’s not easy to break the frame like that. So what else is broken?
Second, if it was a genuine mistake, why wouldn’t he want to make it right? It’s quite easy to narrow down which advert you bought - given what was paid for the pistol, surely a few quid to keep everyone happy wouldn’t be a big deal? A new guarder frame for a gen3/4 is £40 at most. So I’d assume a £40 refund would have kept everyone happy?
If you’ve paid via PayPal G&S - I’d take it to PayPal. Make sure you collect screenshots of the conversations to provide to PayPal as evidence you’ve tried to resolve this like adults. Take plenty of clear photos of the condition of the pistol as well.
Everyone makes mistakes - I’ve sold a couple of pistols that I absolutely know where immaculate but “don't work” on arrival. Even had photos and videos to prove it. However, you could sit around and blame the seller or the buyer, but it doesn’t fix the problem. In those cases typically the seller would either offer a partial refund to the value of repairing the damage. Or a full refund for a return of the goods.
Ultimately it’s a reputation thing. Personally I don’t want to be slated for being a conman or a scammer, so I’d do what I could to keep the buyer happy. The fact he has no photos and he’s taken down the advert raises red flags to me. I mark adverts as complete/sold and I still have photos of sales from 8 weeks ago. If he can’t prove it was immaculate before sending it to you, I’d imagine you have a strong case with PayPal.