The Seocho Flea Market may be the best due to the multitude of households setting up, but Dongmyo certainly is worth some of your time. I've seen this market referred to by different names: Dongmyo, Dongdaemun, and Hwanghakdong. Note that this is not Dongdaemun Night Market, which is a very popular collection of shopping malls.
On good days, you can continue following the lineup of dealers and other sellers east all the way to Sinseol-dong Station, and the Seoul Folk Flea Market. I'll have a full post devoted to that market in the future, which has a permanent indoor space as well as a decent collection of outdoor sellers on the weekend. It seems that the unofficial/official line between the two markets is Nangye-ro, a major north-south road.