The IRS has three classifications for stock trading. As you'll see in: https://www.irs.gov/taxtopics/tc400/tc429, only dealers can be individuals or businesses. An LLC set up under state law as a partnership or corporation can be a dealer.
There are tax advantages for traders. However, even if you satisfy the requirements to be a trader, the IRS considers a single-member LLC the same as a sole proprietor: https://www.irs.gov/businesses/small-businesses-self-employed/single-member-limited-liability-compan...
