This is a risky way to make money, and not recommended. Tell a player that if he beats you in a duel you will give him 10 gold, but if you beat him he has to give you 5 gold. This can be very effective if you are skilled in PvP and make the bet with a trustworthy person, or friends. It is a very risky strategy, as you may lose gold, or the other player won't cough up the money anyway. You will also get a bad reputation if you don't cough up the money in the case of defeat.
The next step on the stairway to the gold cap is the internet. Often times, WoW players forget that various websites can have good information. However, many of them are dangerous and just filled with lies or exaggerations. It is my advice that you stick to three main websites: Wowhead.com, Worldofwarcraft.com, and Mmo-champion.com. On wowhead.com, one can see average prices overall on every sellable item currently in the game. This is a nice start; however, most servers have economies that differ greatly. I played on one server where a +healing red gem went for 100g and another where the same gem went for 55g. Swings like this are just the beginning. Random drop epics, especially those BoEs from newer content sometimes differ by thousands of gold! (Consider this fact especially if you are planning on transferring, as this can be the greatest gold mine in WoW). Worldofwarcraft.com provides a little known list containing the most common bought items on the auction house across every server in an aggregate sense! This means that they average every server before listing the results. Unlike wowhead.com, this presents the reader with a much more accurate portrayal of what they may expect on their own given server. Results will be slightly different, but this tool should not be overestimated.
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.