@Spopssas
I'm sorry that your experience has been so bad. Some of the charities attitude to donations, staff and volunteers can be pretty awful but they're not all like it. Some volunteers and/or employees do use charity shops as a free for all as do some customers to be fair.
I work for one of the big charities and really enjoy my job. We process donations, prep them and then sell them so I do a wide range of stuff. I started as a volunteer and then applied for a paid position when one came up - very rare as it's an excellent shop so it's sort of a dead man's shoes situation with paid employment. The two managers are mainly great and the assistant team really works together well. The volunteers are long serving and mostly great too (you always get some who perhaps shouldn't be there).
Yes, we do get pick of the donations once they're prepped and priced but no one takes the piss and we do pay the same as customers. On top of my paid hours I probably do 4-5 hours unpaid each week which I'm happy to gift to the charity and most of us do unpaid hours every week as we enjoy the job although we are kept short of paid hours we need to run the shop effectively and head office are a nightmare. Generally it's good though and we are doing 'good'.