Like I wrote before, the word "customer" is defined in the dictionary as "a person that pays for goods and services". So going by the dictionary definition, that qualifies me as a customer. The dictionary does not state that only non-staff of a particular shop or business are customers.
The way you describe the way you think sort makes it sound like you think I'm a actively expecting to find things I'd like to buy every time. It's actually not that frequent that something is donated to the shop that I'm actually interested in purchasing for myself. I have very specific interests and tastes. There are loads of nice books in great condition that I don't buy because I have no interest in reading them. That newly released hardback crime novel that got donated and is in like-new condition that I priced at £6.99? No one can deny that that's a nice item, but I don't buy it because I'm not interested in it.
You say that buying books for myself is not part of my job at the shop. Well no, I'm not performing a job when I buy something for myself; I'm taking a break from working to purchase something. Afterwards, I go back to work. I don't know whether you personally work at the moment, but you are surely aware that people aren't literally working every single minute of their workday. Workers have breaks and this is the same in charity shops.
Drinking coffee isn't a part of my job in the shop, but I often have at least three mugs when I'm there and I'll usually have some biscuits too.
The fact of the matter is, the charity itself (Oxfam) has no problem with members of staff buying items on the spot, so while it's not part of our work, it's something that is fully permitted by the charity.
I also don't consider it my task to make the shop look attractive; I've never been asked to do that by a manager. They simply ask me to price and sort things (and are totally happy for me to buy something I like if I spot something) and do the till. The managers themselves will sometimes buy books or DVDs from the shop that they'd like to enjoy; I sometimes see their names on our record of Staff purchases.
Why are you assuming that our shop is presenting "shite" on our shelves? You ask how the shop has attractive window displays - the answer is simple. Because there is an enormous amount of attractive, nice stock that staff do not buy because we have no interest in it.
I do not understand why you think that just because staff sometimes come across items that they'd like to purchase themselves, that that then means that there is only "shite" left to sell. You imply that the shop I'm in is only putting rubbish on the shelves and that is not true. We are putting loads of books of all kinds (in good condition) on the shelves. One such, an item I priced, is currently on the shelf for £8.99 (unless someone has bought it between last Thursday and now.)
When I am buying a book for myself, then yes, I do see myself as a customer first. And even though plenty of the books that I put out for sale are indeed very high quality (great condition, etc) I'm not going to deny myself a purchase if I see a book I want to read. The charity still benefits from the money I pay for it. In fact, some of the books I've bought have been in rather a tatty condition and were certainly not among our nicest items. I've occasionally bought books that were going to be thrown away (because they were too tatty to go on the shop floor) but I liked the subject, so bought it.
While I think that overall, the quality and amount of work I do in the shop is of benefit to the charity, I am not working there because I particularly care about the charity in itself. I am working there merely because I find the work fun. It's a good time for me.