I agree with Jon, you're being scammed.
Basically, I'd give 2 choices:
1; Send the injectors back, he pays shipping costs your way, and when you receive them, you refund his money.
2; Tell him to go piss up a rope, pound sand, or whatever vernacular is suitable for you.
Additionally don't admit anything, your ad tells what's necessary.
As I said, if he paid with PayPal, he's likely going to try to file a claim, so be prepared.
When I had an issue on a deal, the agent asked where the ad was and I sent him a link, he looked at the ad and offered a partial refund through Buyer Protection.
It was my only recourse since the person in Colorado refused to reply to e-mails about a part he sent that was cracked.
Mark.