Sakura Recycle Shop is one of the biggest second hand chains in Phnom Penh, they have five different stores in Phnom Penh and one in Siem Reap. Their shop on Street 271 is probably their biggest shop and it even has a special furniture department. Sakura import all their second hand products from Japan and their shop is nice and organized.
The place is swamped with a random selection of used stuff and at times it can be hard to see the good stuff, but if you have time and you are looking for a bargain then this is a great place.