I haven't really contributed lately...been kind of hard for some reason.
Anyways, this is common sense but perhaps helpful to some less savvy individuals who are new to relisting/resale techniques, along with the abuse of players' inability to check AH prices. Short and sweet, this is a guide to taking advantage of impulse buyers.
Now, as we speak there are some bracers one can purchase with Valor Emblems that sell for 2-2.5k gold on my server. For some reason, people get excited about spending their badges and list the bracers for 800-900g. This is stupid, since I easily sell them within 2 minutes of obtaining my 60th badge, and for 2-2.5k depending on the buyer's eagerness to purchase. What I've done now, is started checking the AH as well, for people who decided to sell epics for less than they're actually selling for.
When I find epics that are lower than they would be otherwise, I advertise in trade that I'm selling them. I'll usually receive a tell quickly. Here's the catch. I'm selling the epics this person has listed at 800g for 2k or more. Even at 1.5k gold that's a 700g profit. I don't buy them until I receive a tell asking how much. If they are willing to purchase the epic, I check their location. Make sure the individual is in Dalaran. I tend to avoid tells from people in other cities since they have the potential to check the AH (sometimes there are multiple epics of the same type on there, if not, you can actually just buy the one epic once you get a confirmed sale, since if they go to view the item on the AH it will be gone). Regardless, once you feel secure about their location/interest, purchase the item for them. Head to Dalaran...sell...make a huge profit. This works nearly everytime if you know your values...you can even go down in price (usually I don't have to). The best thing is if you buy them for 800g and they turn you down, you can essentially resell them for 700-900g anyways, so you don't lose much, or any, in most cases.
Anyways, this may seem obvious, but perhaps it can assist someone who is looking for a good method for getting started up. You can test this with any items you have an urge to try to sell, I use epics merely as an identifier for the more expensive items with a larger profit margin. It would even be ruthless to use this to corner the market on items like ifinite dust BEFORE you actually buy any, aka find a buyer for more and buy it all out. The only problem here is that they're more likely to check the AH first on cheaper, more common items.
Good luck!