Diablo – Do items bought from Kadala have a different stat range when bought in a Torment Game

The nature of the market is such that the value of an item is highly variable. An item may sell for 10 million because someone is searching for just those stats, but the exact same item may fail to sale for 5 million because nobody else is looking for that.

A lot of fan sites do have trade forums. They let you post your item and ask for a "price check". Essentially what you are getting is offers for how much people would pay you for the item. This can get you a sure sale, and it helps you gauge interest, and may even allow you to avoid the 15% fee if you are trusting. But you're obviously limiting your audience pretty significantly, so there's nothing to say that you couldn't get much more by just posting it on the AH.

This depends on what you are trying to accomplish.

If you want to farm for gear, then you should play on either Normal or Torment 1. If you want Torment-specific items, then play on Torment 1, otherwise play on Normal. The farm speed on Normal is so fast compared to other difficulties that you're going to get so many more rolls of the dice that the minor differences in drop rate are easily overcome. You want to play on the easiest difficulty that can still drop the items you want.

If you want to farm for experience, then you should play on the hardest difficulty where you aren't challenged. This will increase over time as your gear and player skill go up, so there's no rule for how to approach this. You will just have to try all the difficulties and see which is the highest one that you can plow through without stopping.

You can't judge anything based on your Damage, Toughness, and Healing, because these equate to entirely different amounts of DPS and survivability depending on class and skill build. There is no rule like what you're asking for except to just try and see what happens.