a lot of LYN traders sell SC2. you can even buy directly from blizz with credit card (wont get the best prices obviously)

what you must check? obviously the legitimacy of the trader, dont want to get con right? and of course the pricing from the trader as you want the cheapest price right?
as a tip, i would advice to buy full retail box packages, as the chances of getting conned is lesser. cdkeys only sales are usually cheaper, however fraud/con are much easier to conduct using such methods. thats not to say there are no legit cd key only sellers, just that its hard to prevent con/fraud cases if you happen to bump into bad apples.

there should be other LYN dealers, as for physical shops, you could perhaps try the various shops at PJ SS2? like software boutique or heroes tavern (not too sure if they sell pc games as well)

or, get both!

and he covered pretty much half the available regions of sc2 ROFL!