Assuming bitcoinstore hasn't ripped off their previous N customers, I'm fairly confident being purchaser N+1, especially if my transaction isn't "special" in any way.
They would have a lot to lose by ripping off customers, vs. a random seller on silkroad, so I'd feel comfortable up to a few thousand dollars.
I think I've actually bought something from Memorydealers before. I'm not sure of the relationship between bitcoinstore and Memorydealers. If they're the same entity, then I'd be fine up to $100k or so -- it's a real business.