Just my two cents, in hopes that it may be of value to you:
1) Having no eBay feedback score hurts you, IMHO. While you may feel the love on this forum, off of it....people prefer not to bid on a car when the Seller has a feedback score of ZERO. You're guilty until proven innocent on eBay feedback.
2) The "rarity" of the six cylinder doesn't really add to the vehicle's value, IMHO. While it may indeed be relatively rare, it's really not desirable, per se, to most people. I personally don't believe that the 250 yields any kind of premium at all. It typically merits a discount in price.
Edit: 2.1) If this car has a Powerglide.....well, it's gonna limit the $$$.
3) Please realize that this is coming from someone who owns a Frost Green '69: the factory color of that car has a very limited appeal. Some may actually like it. And fortunately, the interior is black. But that color gold, brown, and possibly white are probably the three least desirable colors in 1972, IMHO.
4) It may not seem like a big deal, but manual (drum?) brakes are somewhat of a turn off, IMHO. Coupled with the 250, this car was essentially (perhaps not quite) the cheapest Chevelle money could buy in 1972.
That said, you have a very clean, solid, original, unmolested car with low miles and documentation.....whatever the later is worth on a Malibu. Your car is definitely a time capsule. These cars are harder and harder to find. The color is subjective. If the car was born with a 350 motor, TH350, and power disc brakes in the front, I think you'd have gotten closer to your asking price. With a more popular color and these options, I think you'd be at 18K or possibly higher.
Is all hope lost? No....not at all. Not by a long shot. The car is still desirable. But to get top dollar...with the way the car was optioned....it may simply take a while.
I sincerely wish you the best of luck going forward.