What does 'efficient' mean to you? Why is $500+ considered the 'highest possible price tag'?
What are you really trying to get at?
Of course, the 'most efficient' way to sell something for $501 is to sell something worth $502. If efficiency is meant as something you can sell quickly.
If your definition of efficiency means efficient use of capital, then selling something for $500+ that costs you nothing is the best way to go. Does that make it easy? You could probably easily and quickly sell all sorts of illegal things, and for more than $500, and you could steal them, which means you are capital efficient.
So without further context of what you are trying to accomplish, the question isn't very valuable as it stands.
The other thing that comes to mind is, why? You'll get the most satisfaction not from the easiest/efficient/expensive item you can sell, but rather from something you are best suited to be selling.