I'd say it's somewhat of a risky venture, at least if you want fast results...
Like you said, quality is going to make a big difference... Make something cool and somebody will buy it...eventually. If it's super awesome, someone might scoop it up right away... maybe. Heck, you might make something super awesome and no one will buy it... You never know.
Keep in mind though, I'm only speaking from experience selling modded DMG's (non-painted) on eBay, so I don't know what Etsy is like, it may be different/better.
I don't want to dissuade you because I think it is a cool idea! I just don't know if it's guaranteed quick cash... But I say go for it! Perhaps you can get some junk Gameboys for cheap and just use the cases. Then you could sell hand drawn cases, just an idea. PM me and I'll let you know a good place to score DMG's.
I've always wanted to try drawing on a Gameboy before... Going to have to try that with some Sharpies!
Also, check out this rad Gameboy by KOOL SKULL ! Pretty amazing.
