Cars are part of our lives whether we like it or not. If I remain without mine even for a while I feel like I lost an arm and a leg.
Buying a brand new car can be expensive and definitely is not for everyone. On other hand buying a "new" used car on one hand can save you a ton of money, but also can be a real pain to find one that you won't need to put a ton of money for repair somebody else's broken vehicle.
That's why this is important to follow those few very simple rules when looking at a used cars. The best practice would be for you to take the car to a mechanic and pay him to look at it and tell you what is wrong with it. However, this is not always possible and it can be rather expensive. In the US it can take up to $100. Where I am located it will cost more like $25-30.

1) Any kind of smoke coming from the exhaust of the car is deal breaker. The only occasion that there is no problem and the car is having visible clouds coming from the exhaust is when it's cold outside(and you can also see clouds when you breath). If that is the case, you need to take the car to a shelter(garage) to look at it, because it can be masking ongoing issue with the car. There are different colors that the car can exhaust and none of them is good. So this is reason 1 to walk away. Easily repairing such an issue can beat the cost of the car. It's a Deal breaker.
2) The car must always idle well. It means that the engine should not be having any sounds out of the ordinary and should not be shaking. If that is not the case, walk away, unless you are a mechanic and can find easy to fix issue, such as a vacuum leak for example.
3) Now, this is important. A part of the Engine, the transmission is another very crucial unit in a vehicle. We have general 2 types of transmissions - manual and automatic. Anything in-between should not be considered for buying second hand. As I driver I have about 20 years of experience and about 500-700 thousand miles behind my back, I know both types. Let me tell you about them and what you need to look for:

a) manual - first thing, make while you test driving you shift all the gears, from 1 to 6. Try also the Rear gear. Make sure you can switch to any one of them easy. If any of them has trouble to be turned on... This requires a serious repair that cost in hundreds. Most people would ignore it and drive without this gear(of 3th gear is broken, they will simply shift from 2nd to 4th directly) to avoid spending tons of money. Also make sure the clutch is separating normally and it is not too stiff. Worn out clutch is usually NOT a deal breaker, but there are those cars with expensive damper flywheel that is very possible to cost more than the car itself. Normally the manual transmissions are way more reliable, so my advise is to look for manual transmission on a used car.

b) Automatic - repairing an automatic transmission is very very expensive task and there are a very few people that can do it correctly, so if you find a very cheap car with "automatic transmission" issue - stay away. First thing to check, try all the shifting positions to be working -> "Drive", "Rear", "Park" on a inclined road and see if the car can stay on Park without the emergency brake enabled. Next thing is to test drive it and on a highway type of road and see if the car is shifting good. There should be no pushes or impulses during shifting, especially at the higher speeds. The last test you can do with the automatic is to enable the Cruise Control(if it has one) on the Highway and see if the car shifts okay - if it becomes unstable, have trouble to keep the speed we have a problem. Generally, walk away from any issues with the transmission. They are expensive to fix and literally not worth it on most older cars.
4) Check the OIL. Pull out the oil stick - it shouldn't be bellow or near the minimum amount. If it is around the minimum you should know that this car is losing OIL. And losing oil is not something you want to deal with. Yes it can be an easy fix, but it also can mean that the engine is worn out and needs repair. Repairing the Engine is Never cheap. Another thing you have to look for when pulling the OIL stick is what the oil on it looks like, if there is white foamy residue, stay away - mean that the ccar is mixing oil and coolant and that is a hell of a expensive to fix. Also you should be looking if there are some metal shavings On the stick... Not a good sign for the engine and another reason to walk away.

5) Get a OBD2 scan tool to read any errors that the car computer has. There are different kinds of devises you can get. They are not very expensive - for the price of $20-80 you can get a working device that not only can help you when you buy the car, but also when you encounter issues in future while driving it. One of the most common scams car sellers are doing is when they have the Check Engine light on is to disable it in some way - this could mean tons of different stuff, but almost never is something good. But with the OBD2 scan tool you can read the error codes, then put them in your smartphone and search Google for it. And depending on the severity you can either negotiate a better price or just walk away if the issue doesn't worth fixing.

Those are my top 5 advises for everyone who's looking for a "new" used car :)
Looking at them now, if anyone could of told me those years ago, I would save tons of money...
Hope you find this interesting guys.