Cash Tech, Brockley Cross
Number 9 Brockley Cross - the shop on the corner of Brockley Cross and Brockley Road that was formerly a money-wiring office and, before that, a florist - is now a "computer exchange store" called Cash Tech.
They buy electronics off you, repair old stuff or sell new electronics on a "layby" basis (20% cash deposit up front, followed by weekly installments).