Sold + email address?

On one of my accounts i'm looking to sell a personal product of mine, but i'm not looking into creating a website just yet. I see people people's posts and they tell their followers to comment "SOLD + their email address". Am I supposed to email them an invoice or how does that work? I'll be using Paypal.

Also, I already have a business account with paypal, verified and all. How can I make the paypal represent my instagram account?