For lowest prices particularly for popular new releases, retired sets, large sets and collector's editions, Spore's the best.
Some of the private and online sellers are pilots, flight attendants who use their free cargo allowances to bring in the latest releases and the much demanded sets (and watches of course). The only snag is that you need to COD at their Spore addresses. For the more expensive sets, they don't and won't despatch to avoid disputes over loss, damage etc. But man, if like me you like to bitch over box condition, content completeness, they're the guys!