I understand where you are coming from, but I have to disagree (me being one of the people who can afford whatever I want most of the time, of course I would)
Consider this: MR Holy One just sold a Reindrach to me (Note: this is the FIRST creature I have EVER purchased from a player, for coins/credits/otherwise). To Unlock this creature from the MD shop it would cost ~55 credits, and it is only available at one time of year, once per account. I paid 30, his asking price was 25 (I just thought it was worth more than 25).
Consider if someone less financially well off than me had wanted a reindrach. Normally, they would need to spend ~55 credits, AT THE RIGHT TIME OF YEAR, to get it. From this case, they could get it at LESS than half price, at any time of year.
Then, let us consider if we DO ban purchase of things via credits. What do we use instead? Coins, creatures, and items.
Items are almost always worth far more than even a rare creature, and very few people are willing to part with them due to the way they are normally created (wishpoint+materials to create a character specific item). So while it happens, Item trading is not very viable.
Creatures are another case, with them being the main trade good. However, the majority of valuable creatures are obtained via the shop. This means that you are paying credits for them anyway, and as I showed in my above example with the reindrach, it is quite easy to get things cheaper as it currently stands. That means that being able to buy things via credits directly actually helps to make things fairer rather than unfair.
Now we move to coins. How are coins generated in the first place? (excluding Murvine intervention, of course) Via the MDShop, ie credits once again. There are certainly many ways of getting coins, such as gifts, quests, the shop, and trading, and these do serve quite a valuable place in the market, but ultimately they are only created via the expense of credits.
There are a further two benefits of the current system. The first is from a buyers perspective in gaining credits over coins, and the second is another way that this method is, in fact, fairer.
Firstly and quite obviously, credits are a lot more versatile (which says something considering how versatile coins are in the first place). Part of the appeal is that, if you have just gotten deep enough in the MD shop for the Sword and Shield items, if you sell for credits rather than coins, you can then turn those credits into the coins you would have otherwise gotten, with a nice stat boost attached.
The second aspect, which helps to keep things fair, is that players who themselves cant afford to buy credits can sell things to other players for credits. This helps even the playing field between those who have the disposable income to pay, and those who dont.
Now, all of this said, the above only really applies to someone who can't afford to buy credits selling things to get them another way. There is certainly a problem when you have people buying things for credits from others who can also afford to buy credits, as then you have a case of the rich getting richer and the rest of the community has a harder time gaining any benefit from it. This is why I have NEVER sold any creature (I only give them away) for anything, and why I have only ever purchased one creature by any means (although I have made offers in the past, I only make one offer and never haggle).
Overall, while the above is worrying, there is not too much you can do about it without making things more unfair in the big picture.