A lot of charity shops employ people these days to go through their stock and sift out anything that has value on eBay (even the ones that appear to have old dears working in them!). The paid employees tend to not be at the front of the charity shop dealing with customers though. But basically in the majority of the shops anything in the front of the store has already been pre-sifted and pre-valued by the charity shop and they've decided to sell it in store because they need to raise some quick cash to improve their cashflow rather than having to store items and wait for them to sell on eBay.
Thus there isn't really a moral grey area here. By buying something from the shop you're helping them meet their cash flow needs and get rid of stock that would otherwise be taking up valuable space for them. My local charity shops love it when I come in and often invite me out back to have a look at what they've just had in as they're happy for me to take it off their hands and sell it on because they're getting money, freeing up space and they know that most of the profit I make will be spent back in that charity shop again next week.
For me therefore it becomes much the same as buying a product from a wholesaler, they sell it to you knowing you're going to sell it on and make more money but they're happy as long as they get paid.
P.S. I volunteer for a charity as well so am fully aware of what
@Dick Barton refers to in terms of not all of them being squeaky clean!
Anyway back to the point - well done
@Dick Barton on your sales!