I used to spend my Saturdays working in a charity shop in Woolwich for Greenwich Mind. Sadly there was an awful lot of tat that had to go into black bags for jumble sale collection. I don't think there are so many jumble sales these days as things tend to go to boot sales or are sold on E-Bay. I agree that it's ridiculous that things like books are thrown out but unfortunately some people just chuck everything indiscriminately into a bin liner (soiled and all) and leave it outside the charity shop.
I know it is more work but it might be better if charity shops offered to sell stuff for hard-up customers (rather than them needing to flog it on E-Bay). We used to do that and go 50-50 on donations and it improved the quality of the stuff on sale and helped everybody as the sellers also often picked up something they wanted to spend the money on.
As for the bags through the door, I don't think I've seen a genuine charity one for a long time - all seem to be for collection by exporters to Eastern Europe and the like and I think a bit of a scam.
If anyone can recommend a genuine charity that would collect I could find a load of stuff to unload.