Debenture tickets are the only tickets that can be legitimately resold by anyone. Other tickets should only be sold by the AELTC. Development of Wimbledon complex is funded by selling debentures which entitle the owner to two tickets every day to a particular show court for a period of time, usually five years. They are permitted to resell these. Most get sucked up by corporate hospitality but they can be found on various ticket resellers. They are never that cheap.
Unlike the US Open, all access is for a full day; there is no concept of a 'session'. People who leave early have their tickets scanned at the exit, and the show court seats are resold for £10 inside Wimbledon after 3pm from the ticket office on the Aorangi Terrace (proper name for Henman Hill, Murray Mound etc). This as the name suggests is a terraced area, situated behind Court 1, which has a large TV screen showing a match from Centre/1. Money from resale goes to charity, last year £167,000.
After 5pm ground tickets are only £14, subject to availability. Can be a cheap evening of outside court tennis.