Motorcycles are secondary modes of transportation for most people. They typically have low miles, and will last for 20 years with only routine maintenance. People tend to sell them when they need money, and buy them when they have extra money. They also tend to sell for a lower price in the winter than in the summer.
This all adds up to a business that requires cash, knowledge of motorcycles and a keen eye on the classified ads.
I've actually watched somebody do this for the past couple of months quite successfully. He only needs to make $500 to make it worth the time usually. The major hurdles to get over are having cash on hand, and finding good deals on motorcycles. Another trick is to have the seller sign the title, but don't sign it yourself. That way it can pass directly on to the next buyer.