If for something you're about to list you change the payment email address next to the Paypal logo during the listing process.
For something already bought you can send the buyer an invoice direct from your Paypal account using the Send & Request link at top of the summary page in your Paypal account.
If they are not happy paying to another email address you can cancel the transaction (via More Actions, right of item on sold page > Cancel > Buyer asked to cancel) and relist with the correct email address on the listing for them to buy again.
