I'm from South Korea, and although my last visit to Yongsan Electronic Market was 4 years ago, the entire shopping complex is notorious for scamming naive customers. Make sure you know exactly what you are looking for, and look up the price before you go. The people there are known for selling used items as new, and often people are threatened by tough looking guys to make expensive, and likely unnecessary, purchases.
I'm not saying it's full of thugs, but taking caution would be wise. Yongsan electronics market has a nasty image among koreans, and that's for a reason.