As a regular SE commuter, I never buy anything from the trolleys purely because I really begrudge the prices they charge. But that's a personal thing for me - I don't like being ripped off. As a commuter, you often feel as though you are paying through the nose for everything and the trolley is an extension of this. Perhaps a discount could be offered to season ticket holders, or even offer the Bite card discount on the trolley service to bring the prices down to a more reasonable level.
I also agree with a previous poster that the rise of meal deals, food establishments and numerous platform facilities do kind of remove the need for the trolleys.
As for the trolley staff, I've never found them rude. More often than not, I feel sorry for them as they walk through the train saying "Teas, coffees, snacks" repeatedly and being ignored by pretty much everyone. That said, I don't feel sorry enough to buy something from them at a rip-off price !!