Another gotcha, in the UK at least, is that you can't make a quick cancellation of a transaction if you have no funds.

If you make a purchase using PayPal, they are happy to take the funds very quickly from your bank account and make the payment to the other party straight away. If however something goes wrong with say and ebay auction and you need to refund the buyer you can only do it immediately if you have funds in the account.

If you don't have funds then you have to wait until they do a very slow, week long, transfer from your bank.

When I had this happen to me the buyer, reasonably expectedly, got pissed after a couple of days of no refund. I then instead had to pay them the money with a new PayPal transaction (which could of course happen immediately) and lost my chance to get a refund on my PayPal fees for the original transaction.