But charity bookshops have different motivations and appeal to different shoppers.
Essentially it’s only bad if you like reading good, hard to find books.
Or believe that others finding them when they might not particularly be looking for them is a common good.
Support both, and I wouldn't want to give up one for the other.
Also a bit of an aside, but charity shops are also more often part of national or international orgs so a lot of the "gain" isn't localized as the article discusses. Good or bad on a case by case basis. Not sure how it edges out.
It's a common complaint that donations should not be used to pay salaries, but the important question should be how much leverage those paid staff can extract out your donation. Skilled people usually don't want to work for free, but they can magnify a donation by reaching more potential donors via advertising, more appealing shops, etc.