If you sell through Facebook you’re going to get a 1099k to claim the profits for taxable income. Same goes for reverb or eBay.
If you’re selling to a private party and use goods and services of PayPal, you’ll still get a 1099k from PayPal.
If you use zelle, cash, or PayPal friends and family, no 1099k is generated since it’s just like cash. Just note those are not intended for regular sole proprietor business use. If you use it repeatedly for gear sales you do so at your own risk. Still plan on putting them on your taxes. You don’t want to fuck with the IRS.
Also depending on the sales you make, some states have taxable income requirements as well even if you sell online.
Selling privately has its benefits but as always cash is king and it’s best to meet in a public setting when selling for your own protection.