It would have incredibly detrimental effect on trade by a strict reading of the laws
Let's take a look at the rules, shall we?
Also, people are forced to use coins instead of barter to trade in most cases. That is, it is illegal to go into a town and barter grain for meat or whatever. People have to use coins everywhere except between family members or very small villages where trading coins is meaningless. But other than that, you can't sell or buy without coins.
Ah, very interesting. Bartering is illegal, and furthermore, you can't buy or sell without coins. Now, there are two ways to read this. The first, and boring, way to read this is 'You must transact using coins as a measurement to determine the value of the goods.' The second way of reading it, the fun way and the way that will cause immense amounts of problems is 'You cannot perform a transaction unless one side is giving actual physical coins'. Now, why is this a problem?
Letters of credit, that's why. You see, trade is very risky and dangerous pre-industrial revolution, not to mention that carrying a fortune of heavy metals made you a horrifying target for bandits, pirates, highwaymen, robbers, etc. etc. Not to mention that they were heavy and therefore inefficient to transport. Thus, when trading, in was standard practice to use various forms of letters of credit in order to conduct trade as a method of convenience. Except, you know, it's not only a method of convenience - it's pretty much how large scale trade can work. That's part of the reason why banks existed back then, after all - a means of using these letters of credit wherever you brought them.
If you forced every exchange to require the physical presence of coins, than that will slow down trade by an enormous factor, not to mention tie down capitol that could otherwise be used. (Because if a ship full of gold is being transported across the ocean for four months, the owner of that gold can't use it, even if he knows he's going to get it. In real life trade, this wasn't a problem per se - if people knew you had money coming in, they'd be fine lending you credit or accepting an IOU. Basically, trade will still happen, it'll just be greatly reduced.
So, let's say you want to go with the first way to read the law - that letters of credit and IOUs are fine. That's fine - until you run into money. Modern fiat currency is loosely based of letters of credit, and it's a lot easier to transport around. If you're fine allowing things to stand in place of the coins during a transaction, than it's only a matter of time before your citizens decide that gold and silver is too inconvenient and heavy, and decide to start using paper money. (Well, actually it'll start with the rich people who gain the most because they're actually using that much gold. Poor peasants won't have a problem. But the system will eventually trickle down to them - case in point: the history of money.)
Also, having money is great because it means that the supply isn't limited to the number of gold coins anymore and the nations don't have to practice mercantilism. (Yay!) On the downside, that kind of make the wizards sealing of the coin worthless, but it won't be worthless until the system for money kicks in, and that'll take a few hundred years, start to finish.
tl;dr - strict reading of the rules stilts trade, loose reading of the rules means that eventually they switch to paper money.