Not a free market argument but I seem to recall that Roumania has joined the EU and hence accepted the Schengen Treaty. Free circulation of goods and persons between member states, case closed if he's a pro-UE bluff.
But Roumania is a very bad case. I seem to recall that it has a catastrophically bad export balance (please do not throw me out of the window) and it isn't getting any better. Soviet-type Socialism has never been completely eliminated, leading to immense problems: widespread corruption, difficulties in becoming anything more than a sweatshop for Italian firms, widespread poverty, social mayhem (you don't need to go to Brazil or Nigeria to see swarms of starving children living on the streets), inflation (which was wiped clean in 2006, when the old lei was replaced by the new lei at a 10.000 to 1 exchange rate)... perhaps you can help us shedding some light on why Roumania failed to follow in the footsteps of the Czech Repulic and Hungary.
In light of this situation there's small wonder that such oversimplistic solutions pop-up. I doubt that blocking the import of second-hand cars would make Roumania a less polluted place (no offense meant): when Ceasescu was still in power and nobody apart from the "selected few" could afford a car sheep in some areas were litterally dyed black by emissions from plants archaic even by Victorian standards. But the elegant leftists love to hate the car (ie individual freedom to get where you want), apart when you touch their beloved BMWs. It's always other people's cars that pollute. But I am getting long-winded and the hour is late.
A very simple argument is the following: if you put a very high import tariff it's almost certain that the ones you are hitting the most will retaliate by rising their tariffs on your goods. It's a simple matter of retaliation. Germany can live with exporting less cars to Roumania but can Roumania afford to export less goods to Germany?
Final argument: the pollution argument has already been used in both Italy and Belgium to force people into buying brand new cars. I won't go into the details because they matter not here but the result is simple. For a couple of years there has been a sales bonanza (helped by insanely low interests on payments) with dealerships popping up like mushrooms after an August thunderstorm. Now sales have grinded to an halt and car manufacturers are becoming victims of their own greed (now where did I hear this before?). Local governments happily lent an helping hand to their good buddies in the automotive industry while keeping the few but very influential elegant enviromentalist-lefties happy. I hope you catch my drift here.
Yes, it's time for the Dr Goebbels show!