Yeah, I do. Not because they're perfect. But because they're good enough to be worth it. We can devolve to the basic reductio ad absurdum argument, that homicide statutes don't prevent murder either, but we're both mature enough to go beyond that and admit that a law that does it's job isn't going to solve everything it's supposed to, so the question is actually does the law do enough, without intruding too much.
I don't personally see a difference in legal standing between an FFL shopkeeper and John Q Public that means they must be treated differently. While we have so far chosen to, I don't believe we have to. Which I hope neatly skirts the issue of whether the check itself is constitutional to address a subtle but distinctly different argument on whether they can apply to second-hand as well as retail sales.
IF the check itself is constitutional, a question I am also open to, then it is legal for both types of sales, to my understanding. I've asked this question before, and I haven't seen anyone proclaim that either the parties involved or the firearm for sale are legally differentiated enough to make the transaction different with any confidence and cites to back up that assertion.