Buying football tickets from 'agencies'

Hi - I'm in the process of buying some tickets for the community shield game in August. We are down in London anyway and feels like a good way to spend the Sunday afternoon but I'm not a member of either club or a season ticket holder etc so looking at having to go down the 'legal resellers' route.

Anybody had any particularly good or bad experiences out there that might help me? I'm looking at places like ticket4football, ticketbisevents, safetickets and livefootballtickets etc. looks like its going to be around £250 a pop for a decent ticket so keen not to end up on the wrong end of a scam or suchlike!

Thanks very much