Let me explain it again for you, as a lot of people don’t seem to understand business or economics.
First of all, “market price” is where what a buyer is prepared to pay matches what a seller is prepared to sell for. The market for used clothing has expanded in recent years - it is no longer just charity shops, jumble sales and dress agencies but has expanded to include Vinted, eBay and other online market places.
The issue raised in this thread is whether charity shops should reduce their prices below market price (what an item could be sold for in this wider market) because some people (like OP) don’t want to pay that market price or believe that it is somehow morally wrong for a charity to charge the market price.
But if we work through what would happen if all charity shops did what OP wants and reduced their prices below market, we see that it doesn’t achieve OP’s objective (to pay less than she is currently being asked to pay), or satisfy those who think charities have a moral responsibility above and beyond profit-maximising market participants.
Firstly, it would create a very attractive arbitrage opportunity where individuals or businesses buy items at a lower than market price from charity shops and resell on Vinted or eBay at market price, keeping the profit for themselves.
In the short term, it might drive down the market price a little because the arbitrageurs have much lower total costs than charity shops and could potentially price at a lower level than current market prices while still making themselves an acceptable profit margin (price minus costs). Inventory on Vinted at al would also increase dramatically, helping drive down prices. So seemingly good news for OP wanting to spend as little as possible.
However, the long term picture is less good for OP. As most charity shops have a high fixed cost base (rent/rates/utilities/insurance /staff costs/POS fees), most would very quickly find they could no longer cover these costs, let alone generate profit to fund their causes. The number of charity shops would decrease sharply. Arbitrageurs’ access to under-priced stock to resell would therefore reduce, and prices would rise again. OP would ultimately find that she had to pay prices at least as high as she was paying before, and probably higher (as market cost of goods sold would overall increase due to donors no longer giving items to charity shops for free).
As to the moral question, charity shops exist to fund hospices, cancer research, animal rescue and overseas aid programmes by maximising their profits. Their legal and moral responsibility is NOT to keep every potential buyer on the market happy, including those who don’t want to pay market price. (If OP thinks people like her deserve a special charity that exists to sell her cheap clothing then she should apply to the Charities Commission and see what they say.)
As previously mentioned, most people who take donations to charity shops do so on the understanding that they are forgoing alternatives (eg whatever they could earn by selling on Vinted, or landfill, or leaving to gather dust or get moth eaten in their cupboard) because there is moral value to them in supporting hospices and cancer research. They do not donate because they want to personally enrich individuals or businesses who have nothing to do with the charity. If it became obvious that this was happening, donations would quickly dry up, charity shops would close down faster, and overall market prices would go up due to less inventory.
HTH.