User blog comment:Blackendedsoul/Difference/@comment-25926617-20150324124033

Personnally i think that you can use H haki and then you can actually try to use H haki. Obviuosly you can put varying levels of effect on H haki and i think he just didn't see anyone that he needed to go all out for in the auction house that he actually wanted to knock out.

As for the strength thing i think it has less to do with strength and more to do with will and mental toughness and the likelyhood of anyone as strong or stronger than Luffy at the time would have to have some time of mental fortitude so i doubt it.