Don't listen to Mithmee. His biases against GW make it hard for him to offer impartial advice. I think your asking price of $750 USD isn't unreasonable given the number of figures in the army, the quality of the paint job, and the fact that the Steel Legion is out of production.
However, with painted armies it's always going to be a tough "buyer's market." You will have to be patient and look at the competition, i.e. what other people are offering on eBay in the way of "professionally painted" armies. You will see that many of these armies are open-ended long term listings with ridiculously high prices, but what almost all of them have in common is the quality of their presentation. Unsurprisingly, many of them are listed by figure painting services.
You will need to improve your presentation and the quality of your pictures accordingly. Instead of taking selective snapshots of your army, take a picture of the entire army from a "bird's eye" perspective. There will be little details in this pic but a potential bidder would like to see the whole army together. To show more details I would include close-ups of of at least two 10-man infantry squads, a heavy support weapon squad, a 5-man platoon command squad, and the overall army command squad. Not all figures in the army will be shown, but it will be enough to give lookers a good idea of what they're getting.
Also, rather than simply listing the number of figures in the army, break it down and give details. I always list my armies like this, with breaks:
The army specifically includes: 1 x 5-man command squad with colonel armed with powersword, standard bearer, medic, vox-caster and trooper with meltagun,
1 x 5-man platoon command squad with officer armed with boltpistol and powersword, medic, vox-caster and a 2-man missile launcher team,
1 x 5-man platoon command squad with officer armed with laspistol and powerfist, medic, vox-caster, trooper with plasmagun and trooper with lasgun,
3 x 10-man infantry squads with sergeant armed with laspistol and chainsword, 7 x troopers with lasguns, vox-caster and trooper with grenade launcher,
3 x 10-man infantry squads with sergeant armed with laspistol and chainsword, 7 x troopers with lasguns, vox-caster and trooper with flamers,
1 x support weapon squad with one 2-man missile launcher team and two 2-man lascannon teams,
1 x support weapon squad with two 2-man heavy bolter teams and one 2-man autocannon team,
1 x special weapon squad with three plasmagunners, two meltagunners and one sniper.
Etc etc…
In other words, put some effort into SELLING your product. You may still end up with no takers without lowering the price, but at least it's not for lack of trying.
As a bidder an army has to really grab me aesthetically for me to bid on it. I can be very picky when it comes to how well an army's painted (in the eyes of this beholder), how complete an army it is (what additional figures I would need to supplement it with), and whether the conversions in the army are cool or simply look "tacky" (in which case I prefer no conversions at all).
Another thing I would add is a total points cost based on the latest codex. Even if you don't have one you can either get the info from a store copy at your FLGS or with some online research. This way, people will know how "big" it is.