I sure as s**t can compare virtual and real world economics, because, by your own admittance, the 'basics' still apply. 'Basics', in this context, being a fairly lengthy list of rules, phenomena, and behavioral patterns that govern free markets and human interactions in those free markets. The variables present are reduced and the model is not as complex, but it works more or less the same way.
The prices of your grocery store example don't fluctuate as fast, because the management in those grocery stores does not keep an up-to-date registry of their competition. Several other factors go into the price differences of a grocery store as well, such as convenience, quality, customer service, transportation costs, regional taxes, and overall regional wealth of the customers. In FFXIV, all of these are equal; it is no more convenient to buy from retainer X than from retainer Y. Same for quality, etc.
The owners of those retainers can see the competition's price, and adjust accordingly. I gave the stock market example, because prices are generally as volatile on the stock market as they are on MMO virtual economies. MMO economies are absolutely just as exploitable by camping them and "buying low, selling high". As another example, selling during week days will net you lower profit for items than selling on the weekends, because of fewer customers. People can and do take advantage of these price fluctuations, especially in MMOs with matured, fast paced, matured economies like RuneScape, World of Warcraft, and EVE Online. The term is called "merchanting", and you can find articles about it all over Google (example).
Finally, your argument is very much a straw man. You should have read that Wikipedia article you linked. I can recite it almost from memory now, because I pull it out a lot on these forums. If you don't feel like reading it, here's where you went wrong:
"Person B disregards certain key points of X and instead presents the superficially similar position Y. The position Y is a distorted version of X and can be set up in several ways..."
Sound familiar? Oh yes, it's your grocery store example, which while it has some parallels to FFXIV's retainers, it also has a lot of other significant differences which I've listed above. That's what your "superficially similar position" is.
Your red herring fallacy -- which can be loosely defined as introducing a new, irrelevant point in order to appear correct or distract from the issue at hand -- is your comment on retainers. The arguments in this topic have to deal with undercutting, free markets, and generally economic theory and phenomena. The way in which items are transfered between parties is irrelevant for this purpose, especially since it does not incur any additional costs or penalties, and is equal across the board. It is, in every way you slice it, irrelevant.