No "reputation system" was suggested outside of buying from a seller you've already done business with. A government could start with proxy attacks on sellers but now it gets into mass selling of illegal items (assuming they are attacking sales of items banned in their territories e.g. banned books in a police state), and this leads us to the question of how likely any of this is.
No, they would not need to sell anything. They would only need to pretend to sell something. They only need to offer it for sale, and wait for someone to buy. This happens all the time in "real life", where undercover agents posing as dealers make a deal - and when the customer confirms the deal to the point that a judge would accept intent, they arrest the guy. They never actually sell anything. This attack vector is no different.
I also do not share your optimism that a government would never do something that is illegal for regular citizens to do. They do this all the time, and use different words for it. Stealing is illegal. But if you are a government and call it taxation it is legal. Couterfitting is illegal, but if you are a government and call it quantitative easing it is legal. Locking someone up in a cage is illegal. But if you are a government and arbitrarily call that person a terrorist, it suddenly becomes legal.
If your line of defense requires a government (that writes the rules!) to stick to it's own rules without making an exception for themselves, I fear you are making a big assumption that could bite you in places you rather not
A given marketplace wouldn't be on anyone's radar until it is big (say, $1B/year in sales). At that point, an attack might require hundreds of millions of dollars. Can you imagine the dialog of a government agency meeting deciding if it should spend it's budget in this way and who will be the responsible party?
I think this premise is probably accurate. But look at how the government went after the Silk Road. If they consider this a treat, it might not take a billion dollars per year in sales to put it on their to do lost.
I really do not think this attack is anywhere near hundreds of millions of dollars for a 1 billion dollar market. Let's take the lower end of your claim and say that the government would have to spend 200 million in this special project to kill a 1 billion marketplace. That is 20%! That's crazy.
If all they need to do is stop the inflow of new sellers, they would simply offer an arrangement of items for sale. Nobody is going to spend thousands of dollars with a new seller. Most likely, it will be tens or a hundred dollars. That's all the government would risk. Per seller. Let's assume it is 1000 dollars (which I highly doubt). For 200 million dollars, you could make 200.000.000 / 1.000 = 200.000 fake sales a year. You don't need that many fake sales to destroy all trust in new sellers, and you would not have this much money per first sale with a seller with no reputation.
The same questions were raised about governments doing 51% attacks on the Bitcoin network.
This is a completely different argument. I fail to see the connection.