Two kits that are rare and expensive and why:
Legal Problems with royalties:
1. Revell Custeau Society MS Calypso. This is the ship from the TV series. Its a very good kit, but there are issues with royalties that have kept if from being reissued in the USA, not sure about Revell of Germany.
No intention to distribute:
2. Jim Shirley Productions USS Oriskany 1/700 in resin. For many years this was the only 1/700 Essex in SCB-27C fit (SCB-27C is with an angled deck and reenforced flight deck for jets.) Jim Shirley Productions has left production of ship models and isn't interested in reissuing the kit.
Maybe some more:
Canceled:
JAG collective: SS Pacific Princess "The Love Boat" its a nice 1/700 scale resin kit of the the Pacific Princess from the original Love Boat series. The producers of the TV series didn't really research what they were contracting with JAG, and while JAG did what they wanted they declined to issue the kit and a dicast innacurate model was sold in place. Never officially released they are rumored to be avalible occasionally on the secondary (private) market.
Very sought after:
The Revell Victory at Sea set that contained the USS Midway, USS Boston CG-1, PT-109, USS Missouri, and Polaris firing submarine. This kit was issued years ago but has stayed in demand long after it was no longer produced. There was a reissue a few years back by Revell but it to is now in very short supply as it was a limited reissue.
Certain Airfix 1/600 ship kits, The Heller-Airfix bankruptcy incident (google it, I won't even attempt to explain).
Great apprecation long after issue:
Matchbox issued some 1/700 and 1/720 kits in the late 1970s. One kit in particular grew in demand over time. The 1/720 USS Indianapolis has fairly dull detail and its demand fell. However over time the kits strengths became appreciated by scratch builder, kitbashers, and those with access to replacement parts: The Indianapolis kit has a correct shape for its hull, correct shapes for the turrets, and correct shapes for the superstructure. A little photoetch, strip styrene, and replacement parts you have an excelent finished model. The kit is also well engineered to the point you'll never need your putty. In fact I think I am remembering this from an old Finescale article from around 2000-2001.
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Summary: Some kits accrue value over time for various reasons and reissue doesn't keep up with demand or just doesn't happen. This will cause its price on the secondary market (resale such as Ebay, or a few other sources) to be very high.