I don't think goldfish or especially minnows would enjoy going up and down; they like to swim a long way fast!
I still think a drilled pipe covered in moss with the airline running up the middle, so it acted lke a filter, with shrimp and maybe a couple dozen microrasboras would be the best thing.
Sounds like more trouble than it's worth IMO - I guess it all boils down to whether you have the time and patience to maintain a tank that shape/size (the frustration of trying to clean it would be enough to put me off LOL). But if you do keep it, I hope you have a concrete based flooring for it to stand on! Oh, and you'll need a stepladder at feed times
A 2 x 3ft high tank sounds a bit more managable and could sell better than a 6ft tall.
If you're in a hurry to sell it off quickly, I guess there is always fleabay lol - people will buy just about anything on there!