3 ) Finally, how do I know what's valuable? I've seen people sell OS items that I personally think aren't very good
I agree with Pleikki, it's very complex and randomly counterintuitive. One of the things that does not matter at all for an item's price is how 'good' or useful it is. Good example are q7-8 max dmg martial weapons, they have no added usefulness compared to q9. But they are rare and also popular for some reason. Price is always determined by supply & demand, but in gw the demand is not based on usefulness.
OS shield/focus/wand with 2 perfect mods are the most rare, so those will be highest price (adjusted by demand). Then again, OS purple dual mod items are many times more rare than gold, but for some reason very few people like purple, so those will have lower price than golds in general. So yea, general advice is to keep all your OS dual mod perfect gold & purple shield/focus/wands. Post them in the price check section, don't sell in kamadan or to private offers if you get a lot of interest.