Simple, free (other than your time)....
Public Facebook page and Instagram account. For IG, the appropriate # HashTags need to be used. Also helps with the public FB page.
Here's mine for my shift knobs...
https://www.facebook.com/917shiftknobs/
Updating with frequency is key - both sites.
Outside of that, complying with local/state/federal tax laws or licenses. After that, if things go right, as others mentioned, LLC... INC...
My .02c from where I live. YMMV... I didn't sleep in a Holiday Inn Express last night, nor have I slept at all.... TIWAGOS...