So what you're saying is that stores like Microcenter and Fry's fear the competition Goodwill could bring against them?
Ah, paranoia, what a faithful companion in life!
My local Micro Center (where, by the way, I buy ALL my computer gear, except for keyboards - and probably 2000 yards from a Goodwill!) sells low-end keyboards for $3.99. So used keyboards, at Goodwill at that price, or half that price, are certainly competitive.
Cables, at $0.99 are very competitive and that may be the biggest real threat to profit because I suspect that "incidentals" are one place where these brick-and-mortar computer stores really clean up. And monitors, now that thrift stores sell valid working monitors in the $20-30 range, that is a problem. Personally, I would rather have 2 nice 15" flat screens than a single larger newer one.
But for actual computers, no. Thrift stores sell a decade-old XP computer with a 80GB hard drive for $40. Unless it has a really nice case, forget it.