Nobody does that. They just take the stuff up to Goodwill. Seriously, go look there. I see RB/GH stuff there literally almost every day. Some of it is way old, I have seen some RB3 stuff that I stupidly did not buy. Condition varies wildly. I tend to see more Wii instruments, which makes sense given the user base of that system. I see the games there too, but those are harder to get a hold of now because they are out of print and those songs are not likely to be relicensed for future editions.
I saw a lonely DJ Hero turntable sitting up there yesterday, I wonder if anyone will buy that.
Oh god, they should. DJ Hero was WAY better than it had any right to be. I don't even like that kind of music, but DJ Hero kind of got me to like it, at least while I was playing the game.