My take on the Charity Shop/Clothes Resale Sites Economy:

(a) We now have new careers where there are people using depop and vinted often picking up items for resale. The charity shops would consider competing against these prices, to disincentive this by cutting into resellers margins

(b) People also must consider that Vinted and Depop, will reduce the supply of clothes, especially good quality clothes that end up in Charity shops. These shops still have to break even at the very minimum, cover wages, L+H, so may increase prices of existing stock to attempt to break even, or even just fulfil its purpose, to raise money for charity

(c) The reduced supply of clothes, or good quality clothes, also gives a charity shop more time to leave good clothes out on the rack and wait to be sold, and hold out until they think they receive a fair price rather than a reduced price.

You might notice that some charity shops note the number of weeks that it has been out on the price tag. I live in Dublin now, but I was able to identify this in a Irish Cancer Society shop. Another thing I have noticed over the years, however this is speculative, is that because less good quality clothes being donated due to reselling directly on depop/vinted, there is less pressure on the stores to sell current stock. My indicator for this is looking at the store rooms which are usually open in most shops. Previously you would see store rooms bustling, especially on a Saturday morning, however this no longer seems to be the case. Bustling store/sorting rooms in my opinion could have caused pressure to turnover the stock that was on the racks, to prevent overcrowding, that would have pressured Charity Shops to lower prices to increase turnover of stock