Jimmy, is it they won't start or won't stay running? Any vehicle that does not start I would stay away from, unless you can actually look at the car and possibly take an OBD reader with you to read the codes. There are too many horror stories for Goblin builders who have bought cars that don't start, thinking it is something easy to fix, and end up getting another engine. I'm not saying that will happen to you, but it is something to keep in mind.
For cars that won't stay running, especially a SC or TC car, it is usually an issue with reading airflow and/or boost pressure. Turbo cars that have damaged intercooler plumbing will start, but not stay running if the MAF is still plugged in. There are also some similar issues with the SC cars that will cause them to die after starting. These problems are usually fixable.
Ultimately, the physical condition of the engine internals is also a big factor. Any engine with black and burnt smelling oil on the dipstick is definitely a car to avoid. Cars that have low or no coolant could be a sign of an engine that had been run hot. Unfortunately, most times it is a roll of the dice buying a used (and possibly abused) vehicle - getting one at auction is even more of risk.
Be patient and make sure you buy the right car. There are running cars coming up for sale quite often, sometimes from private sellers, that most times gets you a better vehicle. They are usually more expensive, but if you are using the engine and transmission from the donor, can save money in the long run with having purchase fewer wear or dead parts. Your budget will ultimately dictate what you can afford, but keep looking and watch the Classified section on this forum, as sometimes a good car gets posted.
