My learning lesson from P45+ selling experience is (I had waited long enough to sell it for $18K, that's what dealers told me suggested used market price), once you get into MFDB market specially Phase One, you get the value of your used back towards upgrade, as Phase One has attractive upgrade program. I don't know about others, so I can't comment about those brands. However if you want to exit or move to another brand, you may not get a reasonable price.
Also, unless you are well known, people may hesitate to buy from strangers for such a big ticket item. Hence they may prefer to buy a pre-owned MFDB from the dealers, as most of the used MFDB from dealers comes with minimum 6 months warranty. So buyers feels more secure.
Dealers will not usually buy your used MFDB, unless you are doing some further transactions with them. They don't have any motivation to do that, unless they can make substantial profit out of it, and you know the answer.
Even when I was trying to buy a pre-owned IQ140 / IQ160, one of the dealer was not interested to buy back my P45+, as they had enough used P45+ in stock. I went with the dealer who were willing to buy back my P45+ at a reasonable price (better than what I was offered on eBay) and at the same time offered me a good price for IQ160 as well. They even gave me credit for my Mamiya 645 AFD II & 80mm.
I have seen with less than 7000 shutter count and one year warranty, IQ180 to sell on eBay for $26.5K. I missed that deal as I took longer to decide due to some financial uncertainty.The seller's motivation was to sell IQ180 before Nikon D800 ships. He had originally listed for $34 or $32K, I don't remember exactly. He is happy that he was able to sell it.
In my personal opinion, the demand for 40MP MFDB will go down to some extent and it will have some impact of used MFDB market due to arrival of 35mm DLSR. Some of the photographers were attracted to MFDB, not because it is Medium Format, but for higher resolution. At least they were able to sell the close to same size of prints which they were getting from 4x5 film drum scan. Those photographers who doesn't require perceived quality of MFDB or not willing to pay the price difference for it, or prefers high resolution camera with the convenience of DSLR, will definitely opt for Nikon now and/or Canon in future. This will definitely impact used MFDB market or even new 40MP MFDB market. If someone ignores that, then they are living in fantasy world.
In a nutshell, expect very high deprecation (a lot more than normal) for your used MFDB, if you want to sell it in open market. Your investment (if we can call it so) is protected with Phase One, if you go for next upgrade.
We don't have to go too far, we have seen on this forum that sellers who were trying to sell used IQ160 / IQ140 since last 3 - 4 months at their desired price, were unable to sell it even as of today. I'm sure they have waited long enough, however market is expecting depreciation which is more than normal.
It doesn't matter what you prefer or think personally about MFDB, but that is market reality.
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