Was it PayPal?
Many sellers, myself included, will not invoice if using PayPal. Sales under "goods and services" automatically generates a PayPal notification to the IRS of "earned income". You pay income tax on the sale.
Invoice, or not invoice ?
Time to time i buying staff at other popular audiophile online platform. Always asking sellers for invoice to pay them and newer was refused till last time when I tried to buy $100 piece of cable. Before I used to buy lots of staff with invoices by my first request. What’s your thoughts, do you avoid deals with no invoice?