Bitcoins lost due to outdated address I sent a bitcoin to temporary address. So seller did not get it because address was outdated before I sent bitcoins via bitcoins-qt. Is there chance to return my bitcoins?
If the seller, or their payment processor, saved the private key for that address, then they have access  to the coins, and would be able to refund them or accept them as your payment. (If you ever see that your transaction has been spent, you will know that this was the case). But only they can claim the coins; you have no way to get them back yourself, and if they don't want to give them back or credit them as payment, your only recourse is probably to sue them in court.

If the seller or processor discarded the private key when the timeout expired, then the coins are permanently lost to everyone.

Either way, you will have to get in touch with the seller to learn anything further.