Hi
As far as I know do not specify a return IPN address in PayPal make sure that is left unset.
Then ebay and as many other carts as you have can use the same PayPal account.
I have 3 differrent stores all using the same PayPal account and all using different IPN returns.
I have not taken a look at PS to see how that is set but I am guessing PS would pass the correct IPN return to PayPal in its code.
Mike