The shops are to raise money for the charity, other benefits are obviously providing an opportunity for people to get a bargain, and also to avoid waste. I've never volunteered in a shop but know people who have. Charity shops can't always take everything, and finding out beforehand if you have something a bit out of the ordinary for the sort of things you see in there, might be sensible. I was given some golf clubs cos I said I'd try to give them a home and asked at a shop I often take things to and was told that they found that they often hung around for a long time and they didn't have the space. I rang another nearby shop and was told that they'd recently had a regular asking if they had some, and so they'd love them. Win-win situation. Take items needing PA testing to a shop that has someone to do that not the one that hasn't. Or pass it on via freecycle if you can't and the person will come to you.
I personally have no problem with volunteers paying the price anyone else would for an item. There's only so much stuff you can actually fit in your house, clothing has to be the right size, so you're not going to buy nice stuff if it doesn't fit, and why should you have to wait 4 days before being able to buy something off the shelf or rail, when I can go in and buy it that day; that seems a disincentive to volunteering. With the proviso that it is a fair price for the item and as someone said it absolutely ought to be double-checked by someone that it is, and properly receipted. I'm sure the volunteers take advantage of going there to volunteer to take their unneeded goods so they don't need to make a special journey. Say a volunteer sees perhaps a nice tablecloth that matches her decor better than the one she's using, if she can buy it she probably will bring her old one in for sale, which wouldn't happen if she had not got a more suitable one. If something is ebayed would they not be allowed to bid on it for 4 days?