Troubleshooting really comes from a 'feel' for the bike and its systems. It's really tough to troubleshoot a bike over the Internet, because you aren't there to hear, smell and see what's going on. If you don't know what you should be listening for, smelling, or seeing, it can be even more difficult to troubleshoot a bike in person!
A decent way to pick up the basics is to get something like the Haynes manuals
Check these out
http://www.haynes.com/na401.html#anchor1228398
3514 - Motorcycle fuel Systems Manual (Suggested retail price $26.95) - This book covers all fuel systems, from the basic operation of the carburetor and fuelling theory to the most sophisticated electronically controlled fuel injection in use today.
3515 - Motorcycle Basics Manual (Suggested retail price $26.95) - This book will provide the reader with a sound understanding of how motorcycle components function and how they have been developed. Simple terminology is used and the text is supported by numerous illustrations.
4071 - Motorcycle Maintenance Techbook (Suggested retail price $26.95) Standard procedures on how to service and carry out minor repairs on all motorcycles and scooters. Illustrated with over 900 color photographs. Includes information on how to build up a tool kit and keep service records.
3470 - Motorcycle Workshop Practice Manual (Suggested retail price $26.95) Covers all aspects of workshop practice including: use of tools, workshop layout, engine overhaul, repairing accident damage, dealing with engine seizure, removing damaged fasteners, frame coatings and welding.
3471 - Motorcycle Electrical Manual (Suggested retail price $26.95) - Uses easy-to-follow text and line drawings to explain subjects from basic electrical principles to state-of-the-art systems.
A few words of advice. It is very difficult to turn a profit by buying old motorcycles and fixing them up to running condition. By the time you purchase tools and chemicals, and then the parts necessary to get a bike into running condition, you will often have more put into the bike than what its worth. This is not always the case, but it's easer to put money into a bike than you think - especially if you are looking to do a quality job.
Some states have laws prohibiting you from selling more than a certain number of bikes per year without a dealer's license. Operating without a license could probably result in big fines and other problems.
That being said, if you are reselling bikes instead of advertising yourself as a repair technician, you will have less trouble with lawsuits, as you are not putting any guarantees on the bike you sell. By repairing bikes owned by someone else, the guarantee of quality is inherent, and if something goes wrong, it's your butt on the line.
Of the repair manuals and theories, the most important theory is electrical theory. A large percentage of problems come from bike electrical systems. Without proper knowledge of these systems, you could be replacing expensive parts to fix a problem, only to find out that the real problem was just a corroded or broken wire.
The second most important theory (for older bikes) is Carburettor theory. It's one thing to be able to clean a set of carbs and even rebuild them. It's entirely another to be able to tune and synchronize a carb so the bike runs smoothly and efficiently. Too much fuel, and the bike will foul out or get poor gas mileage. Too little fuel and you run the risk of the bike overheating and galling pistons/ burning valves, etc.
You need a CLEAN AND EFFICIENT work area, along with the proper tools. If your work area is not clean and indoors, you run the risk of getting dirt into places where it shouldn't be, losing parts left and right, and losing lots of time because your tools are disorganized. Some mechanics I know spend more time looking for proper tools than actually working on the bike.
Working with older bikes, it is also important that you learn how to change tires. This job is made infinitely easier with a Coates machine or similar tool, but these are expensive. Get a good set of tire spoons and learn how to use them.
Finally, what I would recommend (IF you are really serious) is to buy a techbook on motorcycle theory, and purchase a non-running bike from a scrapyard. Take the thing apart as you read through the books and go through the various systems to find out how they work. Remove and replace the tires numerous times. Learn the various circuits in a carburettor - be able to identify parts without looking in a manual. Learn how to properly use a torque wrench!! Learn how to use feeler gauges to adjust valves...
I could go on. This isn't a little on the side type thing if you don't have previous knowledge or experience. If there are bike junkyards in your area, maybe you could volunteer your time to help out there... Free labor is always a good thing for those guys (Especially because bike repair and breaking isn't a real money-making operation - NOTE THIS), and it will give you a good opportunity to learn the basics without massive cash outlay.