Yeah for anything older than the 3ds and wii u it is pretty much only useful as a cool collection. About the only uses that would not be trumped by emulation would be if you found some beta code written for a dev unit (they often came with more memory so you could build it more easily and then scale it back to work on official hardware-- other consoles might have had faster speeds and more capabilities elsewhere but to the best of my knowledge the Nintendo handhelds tended to be increased memory and VRAM and not a lot else in terms of processing/resources, depending upon how you want to view debugging additions), and even then if you know the differences you can probably tweak an emulator to run it (not all the best ones are open source but there are usually workable open source emulators available, especially for Nintendo consoles).
Some others might have control inputs/outputs so you can test timings or automate tests or something but we see control mods/hacks all the time around here and they are not that hard to pull off (buttons are just large switches after all). Footage capture is nice but between GB players, the SNES stuff, emulation and homebrew offerings for capture it is not as useful outside of the DS and 3ds. Not to mention the DS and 3ds stuff was often a glorified camera rather than signals level like the homebrew capture stuff from what I have seen.
If it comes with the software side of things (not impossible) it might contain some info that ROM hackers might like (the GBA and DS have a whole bunch of formats, many of which have been reverse engineered and are probably more understood by hackers than even Nintendo/the devs that made the formats but playing with a full dev kit could yield some interesting results), and similarly if there is any leaked source code (not aware of any for the GBA, is some for the DS in things like Princess Maker and various smaller things too, older stuff tends to only be things like
http://www.pagetable.com/?p=28 because it was lots of assembly).
You could dev with official tools but devkitpro and the like render that a pretty pointless exercise, to say nothing of the subsequent need for xbox style distribution of any binaries that daxtsu mentioned. I should also mention that the DS at least had a fairly large number of online/phone home type things associated with it, not enough to completely put you off but definitely enough to make devkitpro that much more appealing.
Legality. The dev probably signed a contract with Nintendo to not resell, whether the cards still remain property of
the bank Nintendo I am less sure of. Ebay and co have been known to remove/delist and possibly even sanction people trying to sell such things but ebay play by their own rules anyway (see also paypal and flash cart vendors). You are not going to be raided in the middle of the night for it though.
$15000, probably more than I would pay but as a "get it out of my sight/so I do not have to list 400 different things" type price it is not unreasonable and might even be a good price for a complete collection. Most dev stuff will go for more than a consumer unit but $3-4k new means surprisingly little.
