FOR SALE: 2 x Plana bus tickets from Barcelona Airport to Tarragona Bus Station, Saturday 7th September 2013. Departure time : 13.10 and Arrival time : 14.20. We have changed our journey and now have no use for these tickets. £22.50 is the face value for both. will accept paypal. Soon as payment is cleared email will of e-tickets will be sent over.

This makes no sense to me...why would anybody want to buy 2 Plana tickets from Barcelona to Tarragona over the internet...with no real security in the transaction? And at face value, when they can securely buy them themselves from Plana? Sounds extremely risky for the purchasers, at least.
You might well have more success trying to return the tickets to Plana for a refund?
You could also try selling them on Salou Classifieds on Facebook

They was bought for us by a family member and got the stops muddled up, guess they was trying to help! (they are old lol!)
It has now been refunded!

Pleased that it all worked out fine for you...happy holidays!!