yorkshire I also volunteer in a charity shop, and the answer to your question is no, absolutely not. The only exception is if you notice something wrong with it - eg a small chip on a mug, a missing button etc as these items shouldn't really have been put out for sale anyway. Also, if you were buying for example, 5 photo frames and the total came to £12, it is sometimes worth asking if you can have all of them for £10 instead. Depending on who's serving and what they're like (and I would try and estimate their reaction before asking), they might agree - I have done for things like that in the past, because the shop I help at doesn't have much room for storage, so if we can get a little bit extra space out front then, even if it means a little less money, that can be very helpful. However, do not do it all the time - you'd turn into a CF and the staff will recognise you and groan about you slightly in private after a while when they see you coming 
Fluffy pineapple I respectfully disagree that volunteers are 'cashing in.' Where I help, we are allowed to buy items for a reduced price- the charity has a staff discount that is applicable to volunteers as well. We are occasionally allowed to take things home for free if an item we like is not going to be put out for sale i.e. binned, or to try clothes on (ours doesn't have a changing room) but if we either didn't bring it back or didn't pay for it, we wouldn't be allowed to again. Also, items would not just be binned so volunteers could take them home - the managers would never agree to that.
Although your donations may be lovely, and I'm sure they are, unfortunately not everyone's are- especially for some reason, deliveries of rotated stock from other shops- but all the donations are kept together until they are priced etc so it is easy to get nice good quality looking, donations mixed up others that maybe don't look so great. Obviously, nobody wants a load of rubbish being sold, neither the staff or the customers, so it is important to go through and inspect everything that is handed in. Volunteers 'rummage' through your donations as they must all be sorted, tagged, priced, steamed if necessary, and the paid staff (usually just a manager and sometimes an assistant), do not have the time do all that by themselves. Of course, I'm sure this doesn't apply to every charity shop, or every volunteer, there's bound to be some bad eggs, but I'd like to think it applies to most. So please don't give up on donating to charity shops entirely, we need/rely on donations, esp good-quality ones - but maybe don't donate to that particular shop anymore 
I think something a lot of the general public might not realise is that although charity shops, are charities, they are run like a business. There are weekly and monthly targets: for money they are expected to make, percentages of items they are supposed to sell (eg 5% of sales per week should be books), number of gift aid products sold, number of charity branded reward scheme items they are supposed to sell etc, etc.. The targets vary between shops, and some of them are impossible to make so sometimes things will be fudged slightly (eg gift aid stickers added to a few non gift aided products, or if someone bought something then leaves immediately after handing over their money, putting the item through as a reward scheme product), as if the targets aren't made, the managers will get in trouble. Most managers and volunteers will not compromise on prices as they know there are targets they have to hit - and so they don't really have a choice, except to do as higher levels of management have instructed them to do.