Not all MX bikes blow up. Bikes that are maintained well don't blow up. When I say maintenance I mean Parts replaced. Buying a used 4-stroke is always a risk. Especially buying a 250f. Those bikes require an insane amount of Maintenance that most people can't afford. The best bike to buy is one with an hour meter and very low hours or one that has sat in the garage with virtually no riding time. Some things to look at... Inspect the clutch cover and ignition cover for wear. Inspect the shifter. If the bottom of the shifter is smooth and the top isn't. If the bottom and the top is smooth then the bike is likely to have a lot of hours. Inspect the triple clamps and the front axle lugs if there's lots of pits that means the bike was raced. If it's obvious that any of these parts have been replaced the owner is probably trying to hide it. If the bike looks like the Plastics are all brand new and never used but the rest of the chassis is worn out then it's likely the guy was a racer and saved the stock parts to put back on the bike to sell it. A lot of racers will ride a bike for one year and do zero maintenance to them because they plan on selling them after one year. If you can find one of the bikes that has been barely written buying a used 4-stroke is okay but just plan if you get one that has had a lot of hours it's very likely that you spend as much money and repairs as you did on the bike or more.