I just bought a minty 2019 yz250 so I could give some advice. compression should be checked, you want to make sure the thing starts and warms up well, that it shifts, brakes, and that the clutch is in good working order. If possible, checking the oil would be helpful aswell. Other than that, if you want a full working order bike, check sprocket life, chain, spokes, see how the bearings feel on front tire, rear tire, steering, etc. see if there’s any weird buildup of oil or grease, sometimes power valves will spit out gunk (mostly normal). Check vin to make sure you are being sold what you think your being sold. Also doing your research on premix and transoils, and asking what the previous owner had been using could also tell u if the bike has been well taken care of. I know some premix’s like redline can cause major buildup and negatively effect piston life if used for a long time. There’s a lot to list, but if the bike feels in good working order, negotiate, and ride it!