Before the mandatory background check law such stings on Armslist would be a huge waste of time for little gain (and very few arrests). So while it's possible there are some sting-like things going on now (maybe some of those sawed off shotguns we've seen on Armslist were actually ATF stings) I really doubt it.
With the new mandatory bgc law there will be an impetus (especially at first) to smack a few folk down hard to make an example of them.
And kidicarus is right ... gun owners tend to be a little paranoid about these things (but to quote Kurt Cobain; "...just because you're paranoid, don't mean they're not after you...")

