The correct party to consult would be an accountant; but the answer is yes. Even if you happen to sell one vehicle at a net profit, you are obligated to report the income you've earned. How you report any profits you may earn will depend upon the nature of structure of your business and accounting method you use. What auditors may or may not be likely to "catch" is irrelevant; and considering underreporting sales activity conducted through channels that inherently retain permanent records of the business you're attempting to conduct is one of the quickest paths to potential indictment as exists.
As for sales tax, whether or not you are required to collect sales taxes is going to depend upon the laws of the state where you intend to do business. Some states afford you no discretion; if you sell vehicles that will be registered in your particular state, you may be required to collect and remit sales taxes. Furthermore, any attempt to circumvent sales taxes will subject yourself to exposure to both civil and criminal fraud sanctions.
If you're selling vehicles, you should inherently expect to be sued whether or not the law, or any terms and conditions you may include in a sales contract, ultimately holds you harmless. Any liability attorney will sue first and worry about whether or not a particular defendant may not be liable later. The only smart choice would be to incorporate to ensure any liability is contained to the business assets and your personal assets are kept safe. However, no form of legal business structure can protect you from being sued individually for anything you conducted as a person, even if operating under the auspices of your business. That's what liability and errors and omission insurance is for.
If, even before you've gotten started, you're approaching the endeavor from whatever you may be able to get away with rather than understanding what would be required to ensure you would be in full compliance, you've a long row to hoe for both yourself and any unsuspecting customers. Furthermore, if you choose to sell through venues like eBay, especially if you choose to use commercial merchant payment processors like PayPal, you will also be subject to the terms and conditions they impose upon you to which you agree in addition to any applicable state and federal law.
The best thing you could do for both yourself and your potential customers is to enlist the professional services of both an accountant and an attorney with specific experience in the sale of motor vehicles. Of course, personal experience in the profession of selling vehicles, especially in the management and oversight of a used vehicle dealership, would be even more beneficial both in terms of the confidence it would imbue to potential customers and reducing the risk of legal complications by serving your future customers competently, with integrity, and within full compliance of any laws and the licensing you'll be inevitably be required to obtain.
Best of luck. I hope this helps.