They will either/and 1) attempt to withdraw from your bank - if not 2) hand file to collections who will nag that number incessantly for payment - and 3) (QUITE POSSIBLE) affect your credit record if you used your real name.
If you have no problems with any of that three (and also mail requests sent to your address), then it's water under the bridge for you.
What will paypal do if you have a negative balance? 
