The seller owes it, but when the seller is a non-resident individual the law makes the buyer the substitute taxpayer: the buyer files it and pays it to the town hall within 30 working days of the signing. This is governed by Articles 106.2 and 110.2 of the consolidated Spanish Local Finance Law.
That is why, in these sales, the amount of the plusvalía is usually deducted from the price on the day of the signing. If you hand us the purchase, we file it within that deadline.