Pretty much anything! As long as its HiFi or AV related, it can be traded in against your next purchase. Everything from Cables, DVD Players, CD Players, Headphones, Amplifiers, Turntables, Phono Stages, Streamers, Bluetooth Modules etc... It doesn't have to be working, any condition is fine.
It doesn't even have to be a like to like product, if you are buying a new turntable you can trade in any of the above, working or damaged... it doesn't matter!