It's possible, but really doesn't even matter to me if they did. In the end, whether he was being honest or not about the defective status of the cartridge, I would have done the same thing - refund in full his money. When I started doing this I decided I am not going to mess around with customers and will almost always ask no questions and just take their word and refund what they want. The money I spend on this stuff is so low usually that it just isn't even worth the risk of getting negative feedback. Especially now, at this time, because I am just starting out and do not even have 30 feedback yet. So a negative would pretty much stop my ebay days dead in tracks immediately if that happened.
I think there is way too much emphasis on seller feedback on ebay, but until that changes, I will generally just refund no questions and not risk anything. I am also just like that, Generally believing people are honest and good hearted, and I know if I get something faulty I dot want a hard ass on the other end trying to persuade me out of refund.
It was nice, after apologizing to the man about te cartridge and refunding him within minutes and adding some last helpful words, he responds back and the last thing he says is "I will give you positive feedback when feedback is given", and he thanked me sincerely. And it is nice to see that and know in the end we helped each other out.