Well that depends, where is your location? Most bookstores like Kyobo sell used CD'S and DVDs. However, if you are looking for specifically Kpop, your best bet is online.
What kpop artists are you looking for?
There's a reason why kpop cds are not commonly sold as used. It's because they are all available to download online. But if you are a collector, it's possible to get them cheaper online.