I concur with much of the foregoing but would add...
While I generally buy from my local shops there are some things that I'll buy anywhere and everywhere I find it because of the difficulty in locating. These are the sorts of things that would bring together a critical mass of interest that is best served virtually. I know my LSS operator has said she just doesn't carry much because the interest is lower.
Specifically, ANYTHING Girl Scout/Boy Scout related. There are licensing issues regarding the "official" logo items and I hope one of these days the two organizations will realize there is money to be made in careful licensing (as Disney has). But in the meantime there is product out there but scattered.
Football - particularly aimed at the high school level. Again local supplies tend to be limited and often what is available is either pretty bland - green paper with field stripes on it that could cross over for band, cheering, etc. or juvenile - peewee league.
Theatre - there is very little in the way of paper and stickers that can be used to put pages together on theatre productions. I'm not sure how many unique ways I can find to use the same mask stickers!
I'm looking over what I just wrote and it occurs to me a theme is emerging. The activities of OLDER kids - middle school and high school - are the areas where putting pages together is tougher. So overall, take a look at those areas and focus on the unusual.
But as pas2 said - price, service, shipping costs will also play a determining factor. It can be out of line or the generic stuff will be used in unusual ways.
Deb