Company buying and selling motorcycles on ebay/craigslist?

Is it required to pay taxes on a financial profit made by selling a motorcycle on ebay or craigslist? I am considering starting a business of buying and selling motorcycles, but I'm not sure of the legal ramifications of this. I ume most people do not claim the profit on their taxes, but when you sell 25 motorcycles a year, the auditors may catch on. In addition, if I do start a legitimate company, I would probably start an LLC, but then the buyer would be required to pay sales tax, which may turn some customers away. Is there a high potential for a customer to sue me (for crashing the motorcycle after they purchased it from me) or does a private sale release the seller from all responsibility?