If you guys don't know what you're doing, the best is to buy from somebody who knows and can ensure you will get what you need.
Trying to find the lowest possible price is fun, but you should probably try your luck in the casino first.

Of course there is nothing wrong with asking questions here, that's one way to learn, but I just want to point out that the fraction of the true deals among the too good to be true ones is really small and there are plenty of guys who know how to spot them so they don't last much at all.