My personal opinion....

There is no such thing as "sometimes defective". Something is either not defective (fully functional at all times), or defective full stop (not fully functional at all times). 

At this point in time, I honestly question your integrity, considering the nature of your previous listings. As such, I feel reluctant to provide any further information that could be used to create 'refund proof' scam listings. Not that there really is such a thing.

As above, be clear and 100% honest in your dealings, and you'll be more likely to have successful sales.