Kids that only wear merch from some alternative concert they went to over the previous weekend. Most of the time they are just fangirls or fanboys. They think that their taste in music is better than everyone's else or they want to seem "unique" by wearing merch. Not always scene kids, most of the time they're just douches that think they are cool.
However, sometimes they are actually very nice people that accept all forms of music and wear merch to say "I like this band, if you don't that's cool with me."
I walked into some merch kid at the mall the other day, she was wearing a "Sing It Loud" shirt and a bracelet that said "The Maine". She saw that I was wearing a Iron Maiden shirt and I expected to be attacked with criticism but instead she actually complemented my taste in music and kept walking. Maybe some merch kids really aren't that bad.