GoKawiilWhile it's not good for you as a player, companies had their own reasons to impose region locks (aside from the historical analog display problem where they were physically incompatible). Decades ago, it wasn't as common for games to release globally at the same time, so these blocks stopped people from importing new games early. Different regions have various ratings boards, so localized releases need to be rated accordingly and potentially modified.
It also allows each region's branch (like Nintendo of America) to set appropriate prices for their markets. This is still the case today with Steam; it's against the platform's terms of service to change your location so you can buy cheaper games using another country's currency.
You must therefore be careful of not only what console you buy, but also which games you get. Even if you found a cheap Japanese GameCube, you wouldn't be able to play English-localized games on it. And you can't try Japan-exclusive games on your American GameCube, either. While it's possible to mod your system to remove region locking, doing so is complicated, so it's better to buy the correct one from the start.
Thankfully, it's not difficult to tell them apart in most cases. American GCN games have the familiar ESRB letter rating, while system-exclusive titles have a flat Only For label at the top-left. European games often have "PAL" on the box, while the Only For label is rounded when present. Plus, their spines have a green triangle at the bottom and the GameCube logo at the top. Japanese boxes are smaller and obviously feature that language.
For the console, Japanese models have (JPN) next to their model number on the bottom, along with Japanese text. American models have (USA) after the model number, with PAL consoles having (EUR). Be sure to double-check photos if buying online.