I really do need other peoples advice on this-_-

I started a list about six months ago, with 
people who have ordered things or won auctions 
which have either never paid, ignored e-mails and 
never paid, or have canceled their orders.  
This isn't first time I've had this happen, but 
here is an example: Someone not that long ago 
placed and order for some items from my site, 
then three days later I get an e-mail saying that 
due to another deal falling through they don't 
want them any longer(the person did not tell me 
this before that it was pending on another deal). 
I e-mailed the person back saying alright, but 
wanted to make it clear that they will be on the 
list and I won't take any orders from them in the 
future nor do I want them to bid on any of my 
future E-bay auctions. Needless to say the person 
got testy (using phrase I prefer not to read in 
an e-mail first thing in the morning), and it's 
not the first time I've had this reaction either-
_- Like alot of people though, when you sell 
something and someone orders, you are depending 
on getting that money.

 But anyway, is not allowing people who cancel 
orders with you, or fail to pay, etc to buy 
things from you in the future a really bad thing? 
Personally I don't like to do it, but I can't 
really trust that if the same person orders from 
me again that they'll actually pay next time 
around.
 I try to be as nice a person and accommodating 
as possible to people who buy from me, I gladly 
do layaways, hold things longer, but over the 
last year combining E-bay and on my personal site 
(be it cels or other items) I've had eight people 
who haven't paid me and four who have canceled 
orders. With peoples reactions since I started 
it, and who have been placed on it, I'm beginning 
to feel like I'm overreacting and I'm wrong in 
doing this=(

 I'm curious to know that other peoples thoughts 
on this are.

Thanks^^

Megan.