Add an item to sell as you usually would, then:
1. Just before you list the item, tick the box 'Donate a portion to charity'.
2. On the 'Donate to' drop down menu, select 'Borderline Arts CIO'.
3. On the 'Donation Percentage' drop down menu, select whatever percentage of the sale amount you'd like to go to Borderline Arts.
Once the sale goes through, Ebay will send that amount to Borderline Arts via paypal.