Another note worth mentioning that certain Tamas seems to sell best in phases. What I mean by this is that a month ago P1s and P2s sold as well as the Tamagotchi Connections, but here lately are a very hard press sell. Even if you just stick to Connections, a V1 is next to impossible to sell against a V2 and a V2 next to impossible to a V4/5 (I skip v3 because I don't see them sell well to start with).
Forgive me, but I find this part of your post hard to understand, specifically "very hard press sell" and "sell against a V2 and a V2 next to impossible to a V4/5". Do you mean it is hard to buy a P1-2 at a decent price than before and a V2 at a decent price for a V4-4.5?
Besides that, I have always noticed that the Japanese language Tamagotchis (both the older models and patched colour versions) have always been cheaper than their English versions. I've always concluded it was because there was less demand for them because the market was an English speaking audience. I've mostly used Ebay and it is a different story over at Amazon, where the connection and vintage models go for inflated prices irregardless of the language (I've never really looked up colour models, but they seem to be mostly Japanese-only and at decent prices). The higher demand for English versions does make sense since Ebay and Amazon appear to have a Western-centered market to me, as I've only used the Canadian websites. Having mentioned that, I dislike buying from U.S. buyers because I get ripped off the exchange rate between U.S. dollars and Canadian dollars, as well as a good deal of listings having import charges, and potentially ridiculous shipping charges. The import charges alone will stop me from buying a U.S. listing, unless it so happens that is the cheapest one (which is what I really care about).
The funny thing with the importance of the English versions of a Tamagotchi is that it really doesn't add that much. I have an English Tamagotchi Angel I happened to find by chance in a used lot. Previously I expected that my first Japanese Tamagotchi would be the Tamagotchi Angel. The English used in my Angelgotchi is all token, irrelevant, which I found to be the case when I got my Akai and Hanerutchi. Tamagotchis are notable in how they don't use allot of language at all and once can navigate them simply by experience. All my experience with Tamagotchis both English and Japanese benefited me greatly when I finally got my first colour model, a Japanese Tamagotchi ID. The ID had the same setup as a normal Tamagotchi and some of its traits were borrowed from the older Japanese version (getting money upon evolution, the bursting ball for an excellent game, etc.), and oddly, it reminded my allot of my Dream Town Friend. The only real loss in not speaking Japanese for the Tamagotchi ID is that the dialogue in the park is completely incomprehensible, due to the lack of cues; yet, what is being said might be something so trivial I could just guess.
I do realize the above is a bit of a tangent, considering you are comparing the rise in Japanese prices to the U.S. prices for Tamagotchis. Although, you didn't exactly specify U.S. sellers are selling only English models and not Japanese models. I've already mentioned why I wouldn't buy from a U.S. seller , but another thing I could think of is that buying from a Japanese seller might be more appealing because they are closer to the source. The newer models are entirely in Japanese and thus since no English version may exist for them, the Japanese version is the only one available. In addition, I've only really seen non-Japanese sellers marking up Tamagotchis excessively because they think Tamagotchis are "some rare things that rabid collectors would throw a ton of money at" - such as that one person I found selling six connection models for $100 dollars each.
The other day I was remarking to my brother how I thought vintage Tamagotchis would eventually become scarce because of how the available listings had been depleting. I just concluded I was being paranoid because I'm sure at least 50% of what is taken out of the second-hand market is given back, and the possessions of deceased people also make there way into the market. I do agree with you observation that Tamagotchis sell in "phases", and I would liken that word to "waves" or "seasons". The buying options appear to change around every month, or perhaps every two months, which reminds myself that not all of what I see on Ebay is what is available. At one point I was shopping for a V4 and found, at the time, they were quite scarce. Now I occasionally stumble across a V4 or V4.5 at more or less the price I bought my V4.5. But since Tamagotchis first debuted in 1996, it is understandable that some of the older version will eventually become more scarce as the older they become. Since the bulk of all Tamagotchi versions are Japanese, it is understandable that the once common Japanese equivalent has gradually become the more prevalent version. Perhaps it can be assumed the two have switched places, partially similar to how the English Tamagotchi Ocean is more expensive than its Japanese counterpart.
Having said all this, it is worth mention that the bulk of my experience in the Tamagotchi market has come from Buy It Nows. I don't like buying bidding auctions unless it so happens no one at all has bid on a listing that will be ending soon. The only Tamagotchi I have bought by auction was my Akai, and that had a bit of an interesting story since the shipping was so slow I thought the seller had scammed me. Perhaps due to shipping difficulties on the Japanese side the shipping price had to go up, although I bought from the same seller and I think the shipping wasn't any different.
On last remark is that I've come to realize Tamagotchis are in no way an inexpensive toy. One can expect to spend $30 minimum on a Tamagotchi, and that all changes with the specific version. The ironic thing about this is that my doll collect hobby has a much more expensive price bracket (ranging anywhere up to $250, unless I'm splitting the bill) and yet I consider Tamagotchis expensive. I suppose internally my two price brackets are somehow consistent, although they probably blend into each other.