In the UK, charity shops got savvy to this and now do their own 'mining'. Valuable items go to HQ to list on eBay for a lucrative price, the remaining dross goes onto the shop shelves.
There are some exceptions to this such as specialist charity bookshops which keep the stock locally, but do make sure to price match with the going rate online.
Genuine non-Duplo, non-Megablocks real honest LEGO? Wow.
I've not seen it except by accident (wedged in something) in over a decade, the Goodwills I know all send it to the central area to be auctioned: https://shopgoodwill.com/