Six Million Dollar man is a Bally -35 mpu based game. Common problems with these are burned up power supply connectors, bad rectifier on the power supply, battery acid damaged MPU board, dead MPU board reset section or failed 5101 cmos ram. Any of those will prevent the game from fully booting.
Luckily repairing all of these issues, and more, are very well documented on the interenet (Clay's guides at Marvin's site). The system is so well documented that the guides will pretty much walk you through most problems.
Now, having said that, there is one problem that *can* be a real pain in the a**, a completely dead MPU board with the diagnostic LED stuck on. But if it ever came to that there are several folks that live and breath fixing -35 boards.
If you can drop a note to the seller and get some idea on what the game does, or doesn't do, I can be a little more specific. But in a nutshell, it's all fixable!
Regards,
D