Check Engine Oil
Dirty engine oil is not necessarily a deal breaker, but should be a valid concern. When oil breaks down and starts to lose its effective lubricity and cooling properties, metal parts will begin to wear out quickly. Pull the dipstick and see what you’re buying. Very clean engine oil in a machine that is old can also set off alarms in my head as it could mean the owner has flushed out potential issues. It’s a gamble either way, but worth looking at. If the engine has an oil filter search for it as well to see if it has been changed recently. You might ask the owner if it has been serviced, but you will want to see for yourself to prove the claim.