My new old Powerbook 3400

As with all collectibles, old computers have to be unique in design and among a limited number produced to have any real value 25 years after their manufacture. Unfortunately, the Achilles heel of all collectibles is that their value is primarily era-defined, assigned by the generation that has some connection to the object or device, either from prior use and/or ownership. As that generation begins to age out and their priorities change, the potential market for items that they deemed "collectible" begins to shrink. As an investment, it will likely not increase in value, beyond what you could sell it for today. If you attempt to sell it, the potential buyers would be those who are interested in an early, (pre-G3/G4) PowerPC processor-based Apple laptop, which is a very limited group of individuals. You can search the usual online auction sites and check to see what similar ones are being listed for and if they sell for that price or not at all. A recent generation that has been accustomed to the speed of faster and modern computing would be frustrated at its slowness and incompatibility with new software and (especially) the internet. These limitations render it a novelty that might be entertaining to power up once in a while, but not much more than that. If you have nice memories of using it and enjoy revisiting them occasionally, keep it until your attachment to it fades.