Hi,
Thanks for your post and welcome to the forums. It seems as if you had a few bad experiences that soured your trip. To respond:
1. Yes - if someone approaches you in the street, it's likely they're trying to get you to buy a product or service, even if it doesn't initially look to be so. Not always the case, but much more likely than not.
2. Yep... you get the old "city tour, 10 Baht!" and other too-good-to-be-true offers. You'll soon find as you did that this involves a number of annoying or high-pressure stops via shops selling expensive tat (especially gem shops).
3. It's pretty much the nature of agencies to charge a commission on, say, bus tickets - else how do they stay in business? Commission can vary though, so it pays to shop around, and play one agent off another to get the best price. I always buy my bus tickets at the station, my accommodation online from an international discount broker such as agoda or hotels.com and usually flight tickets online also. The only thing I would tend to buy from a bricks and mortar agency in Thailand is a specific tour, but even that's a rarity as I prefer to travel independently.
4. Thieves exist everywhere... it's certainly good advice to keep your wallet and passport close to your person or secure in a safe in your hotel room.
5. True, as with most things in Thailand you bargain for them. The exception is meter taxis, which can be acquired in Bangkok, and are my defacto mode of transportation away from the Skytrain.
6. Er... yes, I try not to buy anything I don't want

Just more tat to haul around.
7. Yes, again keep wallet and cards close. Leaving them easily accessible or even in plain view is opening others up to temptation. I carry mine on me at all times in my front zip pocket of my trousers. I never leave anything of value in my room unless it's in a safe.
8. Yes, Cambodian visas are best bought at the border for the face price of $20 (which has been the cost for years now). It's true that some agents may try to deceive; but if you do your research, and know the visa cost ($20), then even if they catch you out, alarm bells will ring when you hear the price.
I do appreciate you posting your thoughts for others to be aware of, but none of the above should have been a surprise to you really. It's either common sense stuff ("don't believe everything people tell you", "keep your valuables safe") or stuff you could've found in any guide book on Thailand (or on this forum). Even knowing the above, we can all get conned, sure... but the best preparation for Thailand, or any destination, is to read up as much as you can on the scams, tricks and what to expect. If you arrive fresh off the plane without a clue of what's going to hit you then yes, you will be taken for a ride.
By the way, scams/annoyances/liars exist in Cambodia, too.