Not a manager but a longstanding volunteer and echo a lot of what the OP says.
Any decent charity will have a robust policy for volunteer purchases. In our shop you're not allowed to price anything you are thinking of buying yourself, either the manager does it, or a longstanding volunteer. We do know that for example Kate is always on the look out for 1950s teasets or Sue likes Joules in a size 14. Everything bought by a staff member or volunteer is written in a special book with the receipt number so there is an audit trail.
I don't think it's the policy of any charity to allow managers to speak to staff like dirt. But as with every job, there are some people who aren't suited for it. Working as a charity shop manager is in a lot of ways a LOT more challenging than a standard retail job where at least you know when people are due in and you aren't faced with the situation of a volunteer refusing to do a certain task because they don't fancy it.The charity I volunteer with (Oxfam) has a policy of not accepting any volunteers who are sent there by the job centre. They have to be there of their own accord.
Pricing - it's an art, not a science. Something which comes with experience and knowledge. Most charities will set starting points for items for that particular shop, based on what has sold in the past. In our shop, which is in a fairly affluent area, that;s £3 for trousers/skirt, £5 for a top, £6 for a dress. If it's Primark it has to be brand new or like new to go out, and it would go out at less than the starting price. (So typically £2 for trousers, £3 or £4 for a top). Things like M&S, Zara, Debenhams get the starting price. Reiss, Hobbs, Whistles, a bit more. If something is 100% silk or cashmere, a bit more. Our policy is always to start with the highest sensible price. If you price a 100% silk Monsoon dress at £12, you have room to reduce that if it doesn't sell to £8 or £7 after a couple of weeks. But if you're starting it off at £7, you'd not be doing your best to get maximum money for the charity. But obviously mistakes happen and you get a 14 year old D of E volunteer with no sense of money/value pricing Primark at £10 and Reiss at £2.