Final edit: I've stopped playing the game, and here's my comments for intermediate players:
1) Forget about researching or making items that use more than 3 rares. Rares are hard to come by, you can just as well substitute cheaper items that use less rares.
2) Questing adventures is where the money comes in. Forget about sales, quests will bring back returns many times more than what you can get from sales. Sometimes I'll even refuse to sell an item if I know it is requested by adventurers.
3) Substitute, substitute, substitute. I used to think that only giving adventurers the exact items would bring back rare resources. But I did a study and recorded what I got in return to what I gave. I found that it makes no difference whether you give the adventurer exactly what he wants or a cheaper subsitute, you rate or return of rares was the same. So stock up on items that uses less rares and sub for items that uses more rares. For example, one of my favorite rings is: Ring of Conservation. It only needs 1 gem to make. I use this instead of the Reactive Ring which uses 3 Mithril and 3 Gems to make and the rewards are the same.
4) Other cheap and useful items are: Hurricane Scroll, Devilish Rod, Ionstorm Scroll, Dominus, Soft Potion, etc. All of these use 3 or less rares.
5) Fairly expensive items which don't even use rares are even better: Dark Paladin Hammer, Samurai's Armor, Sniper Rifle, etc. I use these to sub more expensive items that contain rares and they work fine.
6) Finally, forget about which customer it is. It doesn't make a difference. If you don't have the item, offer them the next highest value item that you have of the same class. If it's a ring, offer them the next most expensive ring you have regardless of the customer. If it's a scroll, offer them the next most expensive scroll you have (of the same type or shape. I always sub the Hurricane Scroll for the Apocalypscion and it you get a better rate of return than if you had spent the rares to make it.)
This guide is for intermediate players, i.e. it assumes that you are already familiar with the game and have all 4 craftsman and the thug.
A few introductory comments:
1) As usual, my #1 recommendation for the early and intermediate stages is not to overspend. Make sure you have lots of cash to cover your salary. The most important expenditures in the early stage is to get the other 2 craftsmen. Improvements can be bought at leisure after you get lots of dough, my rule of thumb is not to spend more than 1/2 of your available cash.
2) I think you can safely postpone buying the thug until much later. He costs a lot and I don't see that his abilities change the game that much. More importantly, unlike the other 4 craftsmen, higher level (sometimes much higher) thugs will come and offer their services. He takes a very long time to level up and I always will buy the higher level thug when one comes along.
The intermediate game: The meat of the game at this point is all about selling and suggestions, it's to build up your wealth so you buy all the improvements you want and the items you need to resell or give to adventurers. Many times you don't have what the customers want; suggestions and crafting the most wanted items are the key to this game at this stage.
1) Improve your silver-tongue skill.
2) Research or buy the recipes to the most wanted items or the items that can be most successfully suggested to almost everything. It appears that most of the time, the most expensive item in a category is the one that has the highest chances of acceptance.
a) At the intermediate level, for the sorcerer, the 4 items I sell most are: Vial of healing, Lucky amulet, Rejuvenating herbs and Vial of mana. These seems to be universally accepted for many things I don't have. I often sell the Lucky amulet in place of other accessories and the consumables to non-fighting classes such as healers. Generally, lower classes of potions can be substituted for higher classes of potions, rings can be substituted for other rings, etc.
(Work in progress)