You can shop with piece of mind with BBTS, they always mention if an item they are selling is second hand.
I've used them a few times and they're a top notch lot to deal with.
For newer MPs though I can't go past http://www.premiumcollectables.com.au/
They're a local store with access to most of the MPs as they are released and a lot of members here use them (myself included).