"Had been" indicates the guns were sold before they were confiscated by the police while "have been" indicates that the guns were sold after they were confiscated by the police.

"Have been" would only make sense here if the police were the ones that sold them after they confiscated them which is not what the sentence is trying to say. D is also not satisfactory as it fails to indicate the order of the verbs as efficiently as A.

I like this one because, as is quite often the case with GMAT SC problems, the incorrect verb tenses tend to create an illogical and almost comically-illogical situation.

In C, we get the order that:

1) Police confiscate guns (were confiscated)
2) Later, the guns are sold at an illegal gun show (have been sold)

While, sadly, I'm sure this does happen from time to time in corrupt jurisdictions, logically it doesn't make sense that the police are confiscating guns to then sell them illegally.

D is a little bit better, but as Dublin says it doesn't specify the order as both events are in the simple past tense. And because that's ambiguous, we have two potential meanings:

The guns were sold illegally, then confiscated by police at a crime scene

or

The guns were confiscated by police, then sold illegally

Now, those two sequences are hugely different in what actually happened! In one case, the police did a great job and we need to be concerned about these illegal gun shows. In the second, the police are corrupt and we need to police the police! And since the sentence doesn't say which is which, it's a poor sentence...it may well be worse than not saying anything at all, since in choice D we have no idea who we can trust!