go to events. like, leagues, and tournaments. the bigger the event, the more people you'll find. trade smart, if somebody is asking too much for something, go elsewhere. don't bother trying to haggle most of the time, if they REALLY want it, they'll look for you.
example: one guy wanted 3 base set DCE for his RH pokemon rescue.
later on I found a guy willing to give me 2 RH rescue for 1 base set DCE. sure, in essence I let him rip himself off. he asked what I wanted for a DCE, I asked if he had any RH pokemon rescue. then he asked if I would do 2 RH rescue for my DCE. wants>price.
also, go on craigs list, look for ads where people are selling their old school collections for around $50. (some people ask for $500, so watch out) if they have base set and base set 2 cards in it, they are bound to have 8+ DCE, which are in high demand right now. and trading with players is helpful when wanting playable cards.
some highly playable cards aren't really valued highly by collectors, so in that sense you can come out on top. remember to trade smart and make sure both people are comfortable with the trade. even if it means backing out of a trade before you agree.