Just a quick idea: Make a random sample of recently vendored BoE gear appear on random vendors of the appropriate level range (based on the gear).
For example, someone leveling BS makes a bunch of thorium bracers. He doesn't put them on the AH for one reason or another (some sensible, some not), but instead sells them to a vendor. A level 50 player goes to repair, wishing he could ditch his stupid level 35 bracers, and there he sees it: thorium bracers for sale.
This helps leveling players with the frequent problem of huge gear gaps. It makes it more apparent that other players exist, and they do influence the world. Perhaps not in dramatic ways, but in a noticeable way.