If flipping is an "ongoing trade or business" that you do with some regularity and a clear profit motive, then you report it on schedule C. Because the nature of the work, income and expenses are total different from snowmobile parts supplier, you would have a separate schedule C for each business.
If your flipping is not regular enough to be an ongoing business, then it is a hobby. This changes the tax treatment of your expenses like insurance and utilities, and may change the tax on your profit (gain).
Here is an IRS portal on the question of business or hobby.
https://www.irs.gov/newsroom/heres-how-to-tell-the-difference-between-a-hobby-and-a-business-for-tax...
Here are some Turbotax posts on the topic.
https://ttlc.intuit.com/community/business-taxes/discussion/llc-for-house-flipping/00/2159810
https://ttlc.intuit.com/community/tax-credits-deductions/discussion/tax-issues-regarding-flipping-of...
https://blog.turbotax.intuit.com/self-employed/5-tax-implications-and-tips-if-you-flip-things-for-pr...
Registering as a business is a state matter, we can't really help with. You will be taxed the same on the federal level if you are a sole proprietor or a single member LLC. If you have a partnership with someone else (including a spouse), or if you form some kind of corporation, that can change your tax situation considerably.
