Considering buying a Wii from Craigslist, what should I be looking out for?

Suggest ebay instead of craigslist, and select a seller with tons of good ratings and a return policy, that also uses paypal.

Modded: look for "homebrew". Often adds little cost to the box and saves you some trouble. Once the main homebrew is installed, you can add other homebrew tools as needed; see the wiki site on Homebrew for Wii, which is a volunteer community. Suggest putting on BootMii to keep risks low with homebrew bootup. It makes sure every bootup uses a safe boot memory slot (not exactly, but sort of).

Model: much better to get older Wii from 2007/2008 or so becuz original model (rev 001 I think) had TWO consoles in one - for real, physically- The first Wii was well made and it had a Gamecube inside it plus the Wii console. Yes a complete gamecube processor system plus the Wii processor system, packed into one console very efficiently, and sharing the optical disk drive. Top edge had hinged door toward rear which opened showing the physical cable and memory card ports that Gamecube used. Wii of course used bluetooth & infrared instead. Later Wii's did NOT have this backward compatibility.
....There were many excellent Gamecube games including rpg's. You can alternatively get an old gamecube as well but why not have both complete systems in one console. Some of the non-backward compatible Wiis had the disk slot and label made to be used lying flat on the shelf, and the older ones had the label oriented to stand on edge; but not all, so be sure to check that the seller has the exact originial model and Rev number, and not the newer versions. Also, the older Wii is older software which is more readily homebrewed (if I remember correctly), so look that up too. While you are at it, puck up a nintendo-brand Wii "Classic Controller Pro" which is very ergonomic, as well as a set of gamecube controllers, cables and mem card (it uses a cartridge not sdcard).
.... You can still get the laser or entire disc drive/laser assembly via amazon or ebay, if you ever need repair (see youtube). Commonest problem (not very common) reading double density discs is the laser just needs to be cleaned with an alcohol-dampened qtip, & you can see how on youtube. Sometimes a wiimote gets out of sync (use reset buttons); sometimes the infrared bar gets old/dim (look at it using your phone's camera too see if they are on), and you can still buy those bars cheap if necessary. Stick to nintendo (they build tanks), or Nyko for 3rd party accessories. .... If you have a homebrewed older Wii with Gamecube you will also be able to burn both types of games to an external hard drive... so then you can keep your original discs clean, plus, and this is big, you can run most of tgose games beautifully on your pc using the free Dolphin emulator. Xenoblade Chronicles can be played with Dolphin on an i5 in 720p or 1080p, its a fantastic A+ jprg and beautiful too. Dolphin is great! Any controller or the wiimote can be connected to use on pc with an adapter, no big deal. ... Used Wii & Gamecube discs are usually inexpensive unless becoming rare. The best games are still great game design today, and are worth getting, especially when not beat up. For example, Baten Kaitos (Eternal Wings), Skies of Arcadia Legends, Tales of Symphonia on Gamecube; Xenoblade Chronicles (2011) on Wii, The Last Story, Muramasa Demon Blade, and Fragile Dreams, Little King's Story, Sin & Punishment, the Zelda games, and Okami.

PS - I bought used Wii twice and no problems. One already brewed. Also used PS's. Key is looking for the exact model & software version you want (ask in email, & expect detailed description plus seller should be responsive via email to you), and important to take a few minutes to pick a highly rated seller for used gaming gear, from a website like ebay or amazon which stand by their customers. Used Wii's only cost 55-75 bucks in good condition with cables & sometimes remotes, and may cost less. Also same approach works well with other used consoles. Run it on a shelf, with air around it, not in midst of junk or sucking lint on the floor. Especially if its an older playstation.