You will find that a payment is usually marked as "pending" because the buyer has no funds in their paypal account and do not have a debit card or credit card linked to the paypal account and all payments are made by transfer from their savings account.
This has happened to me many times and it usually takes 3 to 4 days before paypal consider you have received clear funds and they suggest that you do not send the cards until the funds have cleared. You do not have to send the cards until you receive clear funds.
And don't think COMC are any better. Do a little bit of research and you will see that if you want to "cash out" then they keep something like 20%.
I've been tempted to sell a few cards over the last few months but have always done without as there was no way I was going to wait 21 bloody says for them to release my money.
Yeah, it is cleared into Paypal's account, but THEY don't release it to you for 21 days. It is a rule they implemented for sellers who haven't been proper;y verified, or have only sold a small amount in the past.
I get it too, it is stupid, but, as a buyer, if something went wrong, at least Paypal has the money and I will get it back!