I haven’t bought used myself, but based on my own experiences with my printer (MK4; I love it) I don’t think there’s a ton to worry about buying a used quality printer. I would not buy a used low-end printer because the odds are much higher that the seller found it frustrating.
A used Prusa MK3S is probably an excellent choice if a Prusa MK4 kit is out of your budget and a Bambu printer is out of your budget or doesn’t meet your other requirements. The seller probably either realized they don’t actually print often, or upgraded to a MK4 (or XL if they had the budget). While you can upgrade a MK3S to MK4 with Prusa’s upgrade kit, the cost of the kit is so close to just buying an MK4 that it’s not worth doing (and Prusa admits this; they only offer it because of the flak they got for not doing such a kit in the past). The MK3.5 or 3.9 upgrade kits could make sense for some people…but in many cases someone looking to upgrade would likely leave the MK3S untouched and just buy an MK4.
They definitely don’t know what they’re doing. They featured this one, which is a death trap. It has a disclaimer that it might not be safe above 120V, but it’s absolutely unsafe and a code violation in the US, where we use 120V (and are very litigious). The disclaimer says they’re trying to get it approved which implies they believe it could be and that the design is sound, but fundamentally it cannot meet code in the US for mains voltage use.
Even if the design were sound, there are material requirements, and having seen the quality of prints some people find acceptable, there’s no chance allowing random people online to print their own boxes is safe.
I think they basically run the contests and feature things based on “ooh this is neat” and “this will excite people to use 3d printers”. It’s a marketing thing, and I guess I accept it because I have low expectations of even pretty-good businesses. But if it’s illegal…someone should probably let them know.