It seems that most exploits of the merchant are based on multiple account creations to influence artificially the pricing of the cards.
An easy way to prevent this would be to suspend the automatic creation of new accounts and to moderate accounts creations in the future.
There are several factors which allow someone to manipulate the merchant. Removing any one of them would solve the problem.
1. New accounts (if people can't open new accounts, they can't mess with the system).
2. Starting gold (if new accounts have no initial value, they can't be used to mess with the system).
3. Buy/Sell singles (if the merchant no longer dealt in singles, players cannot manipulate the system).
4. Price increase algorithm (if buying cards no longer drove up the price, players could not manipulate the system).
5. No price limit (if card prices were all capped, it would become very difficult to manipulate the system).
That said, the last time I checked, each set only had 0-3 cards that cost more than 100g. Of course, we do now have the other issue...
After the last merchant fixes (cards go out of stock now)
...which is essentially no different than having a price so high no one can afford it. If the Merchant is going to deal in singles, I think it is silly that it should ever be "out of stock" on anything. As has been noted, there is not much incentive for players to refill that stock.
the most some desperate troll could do, is to spend his day trying to buy out all rares out of stock, but as we saw his hard work was undone for 15 minutes just by some people opening some boosters.
Apparently not, since
Mauhur, Relentless Hunter has been out of stock for at least a day now. Which is extra-silly considering it's a fixed starter card, and for some reason the Hunters starters
still aren't available in the Merchant.