well, I jsut tried again.
and firs quick talked through his dialog, got the shells.
and when returning, he jsut told me that one line "Can I interestyou in some high quality gear" pr something.
on the right side, below the gears, I see 9 green slimes (likely those shells you mean) so I should have enough.(thats after fighting the colloseum)
or am i looking in the wrong place?