This is cumulative, as you take into account each failed sale as a loss. In the event too many deposits accumulate, you must decide whether to continue selling the item inflated by too many sales or finding of another way to dispose of it. One final tip: when you find that an item fails to sell in one AH, selling it on the other may finally dispose of it and get you your earnings.
Some people have made arguments that quest blues can sometimes be more powerful than heirlooms, especially with sockets. I'm not convinced. The simple fact that you never have to even look at quest rewards will save a non-insignificant amount of time over the course of leveling. Also of consideration is the value of enchanted gear never having to be re-enchanted.
Guilds are perhaps one the most effective ways of progressing your character, and in turn, making money. Most 'high-end' guilds have a guild bank where members donate items for other members. This may range from potions, reagents, and craftable plans. Usually you will have to donate to a guild bank in order to receive items as well as stay active in your guild, but receiving potions that will aid your progression and craftable plans allowing you to profit off selling the products will benefit you in the long run. Also, donating to your guild bank may mean donating something you cannot use in turn receiving something you can use. Sometimes, additional services such as VoIP servers are provided and play a key role especially in end-game content; communication is paramount to a the success of an efficient group. Efficiency results in receiving gear faster, running more frequently in a shorter amount of time, and in turn making more money from runs. In a well put together guild, members become a close knit community including financial and questing support, which are among the most profitable benefits. If you have not considered joining a guild as part of your strategy moving through the game, you may wish to strongly reconsider.
Some players even turn this into their profession by systematically "plundering" vendors in the game world and then selling the items on the auction house at a significant markup. The reason why this works (even for items which are on unlimited supply at vendors) is, that many players don't want to spend time traveling to specific vendors to get hold of a recipe or skill book. They would rather pay a slightly higher price at their local auction house. In some sense, they use the auction house as a "super market" or "convenience store". So it is completely reasonable and legitimate to be the supplier for this convenience store and make money out of it.
If i start on a new realms, I am farming a lot of transmog from classic and Burning Crusade Dungeons and Raids. In the beginning, i start to post them if they have at least a DBRegionMarketAvg (Average value of an item on all EU realms of the last 14 days) of 500 Gold. All the other items get vendored. The time you hit a stock of 1000 Items, my value treshold raises to around 2.500 Gold. All the items below that value get vendored aswell. Newly farmed items that are above these treshold will be listed. Why? If I start over somewhere, I want to get a certain amount of gold quickly. Cheap items are great for that. Later on, to save time, Quality is better than quantity. As mentioned above, re-posting the items on the auction house takes quite a bit of time. At this point, I can recommend you the TradeSkillMaster AddOn, it makes your auction house work really a hell of a lot easier.