Maybe the planet based their society on WWE broadcasts
In that unusual situation, Starfleet regulations require that the Prime Directive be disregarded in favor of the Genocide Directive.
Said prototypes though would negate the need for traditional trade in this regard. Just beam it up when no one will suspect it, scan it into the replicator database, and then return the original object.
Granted, I don’t think that would go into the standard replicator, as that seems to store compressed versions of items, as people say it still doesn’t taste exactly the same as the real thing. The patterns in the replicator do not seem to include the minor imperfections of the real thing.
You can thank the Heisenberg compensators for that! 
Seriously, who wouldn’t prefer an original anything over a copy, no matter how accurate? There will always be a market for such things as handicrafts, foodstuffs, relics, and other artisan works.
Except Data’s egg painting.
A blind man teaching an android how to paint…
There is a DS9 episode where a Maquis member tricks Sisko into rescuing some of his folks. It based on the Richard McKenna Novel and Steve McQueen film, The Sand Pebbles.
The fellow explains to Sisko how much better a real tomato tasted than a replicated one. This may also explain the success of the elder Sisko’s restaurant when food may be easily replicated.
Do we know whether or not replicators are a standard household item?
We had Mr Fusion by 2015
In “Errand of Mercy” Spock passes himself off as a Vulcan trader with the Organians, who at this point look pre-warp, and no one blinks an eye.
Just knowing that advanced star-traveling aliens exist is plenty of interference. But I suppose there would be few examples of cheap shows reusing sets if the PD was really enforced.
Which is why I’m sure there is a black market. But I think the Federation would at least pretend to be above that sort of sentimentality.
Plus I personally don’t care all that much about having the originals of things, and there are probably a lot of Federation citizens like me. Especially since any originals would, like now, probably be rather expensive.
As for how the black market works, I suspect that most merchants would rather not give away that they come from space, as knowledge in negotiations is power. If one person comes from space, there might be others, and the merchant may wind up in trouble. So I’d guess they’d do the Prime Directive thing and pretend to be locals. And I’d suspect they’d only trade in things that would fit in on the planet, so as not to arise suspicion. I don’t think you’d have to worry about someone trading, say, a phaser for access to some dilithium deposits on the world of a caveman civilization. Heck, I’d expect very little technology to be traded, as commodities would be easy to supply and easier to hide.
I don’t remember for certain, but it’s possible that Spock knew the Klingons had already revealed themselves. And the Prime Directive clearly doesn’t apply in full if contact with warp civilizations has already been made.
And that concept gives me the odd idea that maybe some in Starfleet clandestinely encourage others to break the Prime Directive so that Starfleet can come in and claim they aren’t violating anything.
(Bolding mine).
I really think they messed up here in DS9. It would have quite an excellent episode in if they needed to setup a base on a pre-warp planet for strategic reasons and had to grapple with the PD.