If you plan to use nanoloop mono, you should look for a DMG. For everything else, gameboy pocket is fine with some EQ and denoise/notch filter, imho.

back when i wrote gameboy music, i recall always preferring actually writing on an MGB (gameboy pocket) for the sake of portability/feel, and then bouncing the finished track to my computer from a DMG since it obviously sounds much better.

100% this.

First and foremost, if an online listing doesn't show pictures of the back, ALWAYS ask if it has the battery cover before buying. I personally ask if it has the ORIGINAL cover as many of these systems don't come with a cover at all or if they do, it's a replacement made from a different color of plastic. You may not mind having no cover but it can be an inconvenience if it doesn't.

Second bit of advice, buy from a Japanese seller. I've gotten the majority of my collection from Japanese sellers (who sell on the US eBay site) and they're usually in great condition, work perfectly, and they take a ton of pictures with the listing. Rule of thumb is if they have less pictures then they probably have something to hide!

And as a matter of preference, I would go for a DMG rather than a GBC. I just like the sound better, but either will work with most of the same peripherals - although a DMG link cable is usually harder to find than a GBC one and if you buy one of the clear color ones that link to both, they can often times be duds, from what I've seen.

Thanks a lot!

that's if you're at the fleamarket obvs, although a decent ebayer will send you a photo of the battery compartment I would think.

and the volume control thing doesn't matter so much  if you intend to pro-sound mod it before the volume pot

some ideas:

check in the battery compartment that there's no signs of batteries having leaked (orangey-brown goop everywhere.

check the cartridge slot for rust.

take batteries and test it if they'll let you - if there are horizontal lines across the screen avoid, if there are vertical lines you can usually fix it if you have a soldering iron.

If you can take a cart, or they have one check the sound via headphones to see that a) it's in stereo and b) the volume pot isn't really, really crackly.

So help me out if you can here; I'm surfing eBay and local e-fleamarkets, but I don't know what to look for and what to watch out for when buying a GB or a GBC? Thanks.