We can't tell you any of that here. We can answer income tax questions.
As a self-employed/small business person, you will report your income on schedule C. You can deduct ordinary and necessary business expenses (like listing fees, shipping charges, credit card service charges) and you pay income tax and self-employment tax on the net profit. You will probably have to make quarterly estimated tax payments to the IRS to avoid late or underpayment penalties.
Here is a list of links.
https://www.irs.gov/businesses/small-businesses-self-employed
If you have employees you must get an employer ID number from the IRS, you can get it online in a few minutes. Even if you don't have a employees, if you get an EIN, then you can give it out to clients and vendors instead of your personal SSN.
There is no tax reason to become an LLC or S-corp. There may be other reasons but you would need to see a professional advisor, probably an attorney. I don't know if you need a business license, you can check the Texas secretary of state's office. You probably need to collect and remit Texas sales tax from customers in your home state, and maybe all 50 states (although that is not entirely clear at this point).
