You dont have to be market savvy, just market conscious. You do a quest that offers you a tank hat a melee hat or a ranged hat and you play caster? Look at the market(before you turn in the quest) and see which you could sell for the most. Take that reward, and sell it. You send a retainer on a quick venture and they bring back something you cant use? See what it sells for(checking sales history takes like 5 extra seconds and is very worth it in the long run), and list it if the price is good. You fish up maps sometimes, why not clear them and see if the rewards are valuable? The piesteskin map is only level 50, and theres still a couple really nice items you can only get from there. Also, all maps that you gather are timeworn(and maps that you find while clearing other maps are not) - its just the naming convention they went with. If clearing the maps isnt currently an option, sell them as you get them. Even if you only get 10k per map, you can fish one up every 18 hours and an extra 50-70k per week will get you closer to that house. And while i know you said gathering for gil wasnt your thing, but there are a few fish that sell for a fortune too. You dont need to know why people want a hq assassin betta(they can desyn it and maybe get a rare minion), just how much you can make for it.
One more option i didnt see anyone list. There are some crafting materials you get by clearing raids/trials, the first of which show up at level 50. Most people farm these by outleveling their prey and unsyncing the fights, but some of those mats still sell for a lot and this is another way you can fight to earn your money. Several of the level 50(and higher) dungeons also have crafting materials or minions hidden away in chests, and if you get them you can sell them off. The main problem with your original plan is that NPCs will never pay you what an item is actually worth. You need to go to players to get fair value for your work. If i were to craft the highest level suit of armor i could make to sell to an NPC they might give me 2k for it, while the materials to make it would cost dozens of times that. But a player who liked the look or needed a quick upgrade might pay me hundreds of thousands for it, possibly allowing me to turn a profit.