Flippers buy from Goodwill and sell items on eBay and Amazon. That is why the prices are so high on jeans and other stuff that might be designer clothing.
Buying electronic stuff is kind of iffy. I bought mice and keyboards and cleaned them up and they worked. Bought a radio with an extension cord and it worked with some cleaning. I have appliance repair books in my basement in case I have to repair things. I could make money buying broken toasters at Goodwill and selling them on eBay after cleaning them up.
I donate cash to Goodwill because they have a program to give disabled people jobs.