It's not just electronics, but anything they think they can get more $$$ for. I've heard complaints on-line about how posters' local stores ceased having anything worth bothering with after the advent of Shop Goodwill.At some point they realized they could make more money by putting electronics on Shop Goodwill
Locally, it's not quite so bad, but I've heard complaints from people who do reselling (buy cheap in thrift shops and resell for a profit). Although they also say that stuff can and does slip through (particularly in books).
Of course, it can be argued that Goodwill should benefit from on-line sales instead of private resellers. Although it could also be argued that reselling can give private individuals an opportunity to earn income they might not have otherwise.