As somebody who retired from USPS your dead wrong. First of all the Post Office Insurance is a Joke. The individual post offices no longer handle claims. You have to call a 1-800 number which will take MANY MONTHS to resolve. There are also very strict SIZE and WEIGHT restrictions at the post office so you can forget about sending larger speakers or Amps. There are also Insurance limits at the Post Office. Have you done any homework???
EBAY and Paypal now report to the IRS so the Seller of an expensive piece of gear is going to get a 1099 Form the following year. Why would the seller want to lose 10-15% to EBAY Fees anyway???
The Post Office has also closed more than 100 Distribution Centers since I retired. Priority Mail packages can still take 1-2 weeks to arrive as Opposed to FEDEX or UPS whose Ground service only takes 5-7 days to go from the East Coast to the West Coast. So it gets to its destination FASTER and CHEAPER using those alternatives. Everything about your scenario is wrong from a practical business standpoint (and safety)