My club has had bits swap events where you go through other's bits boxes and trade for stuff, kind of like we did with Star Wars figures when I was a kid.
No shops have had bits bins for trade/purchase, but the owner of a store that's been defunct for years now used to let players buy stuff out of his personal collection of bits. I used to get shields for my 2nd ed marine army from him back in the day when they could have combat shields.