I'm not defending the GTHQ. There are problems. Right now, I'm watching a little wheel spin around on an item I clicked on to buy, because the GTHQ has stuck.
But I've found some ways to maximize, at best, a bad situation.
1. You need good wifi. I have way more success at work than I do at home, because my home wifi and internet provider just suck, to put in plainly. I hardly ever try to buy from the GTHQ at home unless it's really early in the morning or really late at night.
2. You need fast fingers. Go in knowing what you want. Don't browse. Seconds are precious. Find what you want. Click on it, and click fast as soon as it appears in a city's trade center.
3. You have the best chance of buying the first eight (on my iPhone, your mileage may vary) items you see. This is related to #2 - seconds are precious. If you have to scroll along to find what you want. it's already too late. You might be lucky if you're looking for a low demand item Forget it if you're looking for expansion items.
4. You need to be patient. Don't expect instant success. Expect crashes, freeze-ups, and lots and lots of sold out items. Remember, you're competing with literally millions of players. Set aside some time to just shop the GTHQ.
I have been successful, relatively speaking, on buying what I want. I'm buying expansions about once a week or so, but that's expanding as I'm up to needing 12+ of each item per expansion. But it's doable. I'm a slow builder, building up not out, so I have plenty of room to build. I'm at level 35, just over 500K pop, and close to 600K simoleans in the bank.