not from the point of view of the charity. a cancer research charity shop (for example) is there to raise money for its own work, not give customers (who may or may not be poor) cheap stuff.
it sounds harsh but their mission isn’t to provide clothing for needy people (or people who aren’t needy but don’t want to pay £6 for the handbag they want), it’s to raise money for cancer research. i’m sure there are organisations that do focus on providing clothes to those in need but most charity shops aren’t there to do that.
from the few months i spent volunteering in a charity shop i can only think of one time someone who clearly didn’t have a lot came in to buy something he looked like he really needed - a possibly homeless man came in to buy a belt as his only belt had broken. we did give him a low price for it, not at his request, and after he paid he emptied his pockets and put about £2-3 in 1ps and 2ps in the donation box 
