Transactions at the legitimate half price ticket booth take place between 10am and 6.30p.m. The choice of shows is printed on laminated sheets slid into a freestanding zigzag signboard to the side of the booth. These are removed at closing or as all seats are sold. Show titles crudely chalked on the signboard itself outside opening hours should be ignored, as should the gentlemen standing by the signs. While sellers may well be legitimate, and abiding by the trading site's purchase codes, you have no way of telling who they are.
Prices are also very high.
Theatremonkey does not endorse buying from these sources. It does not sell tickets, and does not participate in any form of online auction. Please do however use the data on this site to make up your own mind about each deal. On a short visit to London it is tempting to pay anything for a ticket to a SOLD OUT show. If you are desperate, a tout or scalper promising seats is inviting.