When dealing with much older vehicles, I've discovered an odd list of possible red-flags if it's a person-to-person sale. These things aren't rules, but rather guidelines.
1. Does the oil look like its fresh out of the bottle clean? If so, insist to leave it running for at least 20 minutes after the test drive. Afterwards, check that oil again. If it looks the least bit milky or foamy, walk away.
2. Is it a salvage title? This isn't always a deal breaker, but if it was salvaged due to flood damage, it is a deal breaker. Walk away from flood damage. Otherwise, get all the documentation/receipts of when/where it was rebuilt. If these cannot be produced, walk away.
3. Be prepared to spend 20% more than what you paid for it for repairs. Regardless of what is said or promised, I find that the 20% rule is a good way of putting the actual cost into a realistic perspective when buying used.
4. Drive it like you really would and make the owner ride with you on the test-drive. Use your gut instinct. I once test drove a 4x4 truck that a guy was trying to sell me. I asked, "Does the 4x4 work"? Sure, he said. I drive into a grassy flat area and engage the 4x4. I start driving around slowly and he suddenly goes "Easy Man!". I'm thinking, that bump was a whole 2 inches high...... I give it some more gas and guess what? Yeah, the 4x4 engages, but the front axles where shot to hell and just slip and grind. He then tried to accuse me of destroying his truck.....seriously, some people.
5. Don't fall for the little-old-grandma car trick. Yeah, it may be in grandma's name, but the teenage grandchildren have been joy riding it at the local rock quarry for the last 2 years.
6. Always be super attentive of pops and noises in the front end. These sounds need to be addressed immediately. There is nothing worse than driving down the road and a front ball-joint breaks. You will wreck. Spend that 20% on going over the front end with a fine tooth comb.