I think you did the right thing, Megan.  If I had 
an online store or ran auctions and someone 
canceled an order after I had held the items for 
them, turning away other people, then they would go 
on my blacklist.  You shouldn't feel bad or let 
them guilt you. 
I love what Anime Taro is doing with their public 
black list.  That is the most ingenious idea and 
helps to warn other sellers not to take orders from 
those people, cause they will just turn around and 
cancel (though Taro gives people 2 chances, I 
think.)