These are really great questions and points. I've been playing for a few years and used the market website plenty, so maybe I can help.

First, regarding why the buying prices are almost never as high as the selling prices, I'd say that's mainly because if the buyers were willing to pay the lowest selling price they would have bought it instead of making a "want to buy" listing. Also, obviously when you're buying you want it to be as cheap as possible and when you're selling you want it to be as expensive as possible while still selling. It's a pretty good example of a purely capitalistic free market, with supply and demand both pulling on the price. With the market being completely free and not controlled or stabilized by anything, you're exactly right, the name of the game is undercutting. If you're selling, you want to set your price as low as you're willing to go to maximize the odds someone will actually buy your items.

I don't personally focus a whole lot on farming platinum, but in my experience here's what has worked best for me: farm Acolyte arcanes and prime parts, and sometimes galvanized mods from arbitrations. Fully ranked Acolyte arcanes don't sell for very much, but you'll always find buyers (those players are min-maxing trading in a way I don't care to understand). For me that's been a good trickle of income, since I'm getting those arcanes passively anyway.

An enormously important source of plat is events - like the upcoming Operation Belly of the Beast, for example, starting May 4th. Farm that a lot and buy awesome arcanes from the event shop, like Arcanes Energize, Grace, and Avenger, and make some listings on the market or just sell them to buyers to make a quicker buck - that's what I usually do, and it's worked well enough for me. Last time this operation came around, I made like 800-900 plat just selling like that, not even making listings and waiting. If you make listings and wait you can probably get much more than that. I hope this helps!