I have never sold anything, but have bought quite
a bit online and I see nothing wrong with what
you're doing.  When I place a bid or an order, I
expect that if I end up getting a bill for that
item, I will be obligated to purchase it.  If I
don't, I have wasted the seller's time and perhaps
money and I wouldn't blame them a bit if they
refused to do business with me again.  

Since you didn't put a link to site/auctions, I
don't know if you do this already or not, but put
a notice on your site/auctions stating your
policies and if someone complains, point to it and
say "you agreed to that when you placed your bid/
order".

Here's an example (from animechaos):
Q:  How long do I have to pay? 
A:  Domestic orders have 10 days. 
      International orders have 14 days. 
      Paypal payments must be made within 3 days. 
      If the payment is not in my hands after the 
given time, I 
      will place your items back up for sale.  
You will NOT be 
      notified.*If you need more time, simply 
ask.  :)  Customers 
      who fail to pay may be banned from making 
future purchaces.