Hi Rhys
What you will need to do is contact Shoreline to let them know and to make sure it is ok 1st, as they will have to re-email the ticket and/or change names on the current ticket (as we need to print out and present at the event).
There may be a £1 admin charge to change names too.
If your ticket is for the blue side you may have more luck selling for face value, as blue ticket sales have now ended. If it's for the reds, then tbh you may struggle imho to get £50 as these tickets are not yet sold out.
HTH
matt