The following tips work only in RP servers. Why? Because people who RP like to dress up their chars nicely, it's part of the fun. They'll wear items that have sucky stats or no stats at all, as part of their outfit. This concept is somewhat hard for raiders or PVPers to understand. But it's worth good money.
I'm not offering tips to make thousands of gold per hour, but if you simply watch over these items, buy cheap and sell expensive - you will make a little extra gold without having to leave the auction house, get involved in undercutting wars, pay high deposit fees etc.
Here's a list of items that:
1. Are not very good or beneficial.
2. Look good, therefore - people do pay a lot for them.
3. People who don't RP, don't know what they're worth - so they auction them for too low prices very often.
4. Relatively hard to get: they're green world drops and cannot be farmed too easily.
Pretty Dresses
Watch the AH regularly for these, you'll surely snag a few cheap ones.
Gossamer Robe - a white dress that female chars like. It's green and has no good stats, so it sells for 2-10g on the auction house by people who don't RP. But RPers pay a lot more for it. On Argent Dawn - EU, the most populated RP server, you can sometimes buy it on the AH for 5g and resell with easy for 100-200g. It works.
Elegant Robe - Pretty yellow-golden dress. Sells for 200-500g usually.
Robes of Insight - an item with a very revealing design. Looks a lot like princess Lea's golden bikini. It's epic but many levelers prefer their heirloom chests, and twinks prefer stuff with stamina. On normal servers this would sell for 100g, but on RP servers - expect at least 500g.
Aurora robe - looks like another common 5g worth of a green? You're wrong: it's a pretty white dress. If you're on a medium populated RP server, snag it and reauction for 100-150g easily.
Silver-Thread Robe - Same as the aurora one, but black and sleeveless. There's another similar version named Black Velvet Robe which is a blue, therefore is sought by twinks AND RPers and tends to be more expensive. Again, worth around 100-150g on my server. And players who don't check out the dressing room auction it for 5g once a week
Venomshroud silk robe, hibernal robe - low cut green dresses. Sell for around 100g.
Arachnidian robe - same as above, only nicer colors. Sells for 100-200g.
Chan's Imperial Robes - A hard-to-get blue from a rare in Hinterlands. Same design as previous aforementioned robes, only orange (looks like an old Naxx40 dress). Sells for around 300g from my experience.
T-shirts!
Wherever there's RP, there are always low-level characters (they allow anonymity and popular among ERPers). Hard to dress them up since most gear in WoW has level requirements. Hence, shirts are a popular, easy choice. What people don't know is that t-shirts are sold by vendors on every city for around 6 silver. White, gray and brown shirts - which YOU can buy off a vendor for 6 silver - will sell for 3 gold each on the auction house.
It doesn't always work, but the deposit is just a few copper so not much is lost, right? I know it's nickels and dimes, but when I start a new char on a new RP server, every bit of gold helps. If I manage to make 10-15g a day by selling shirts with a level 3 char, why not?
Sexy breastplates!
Check out the design on these:
Vanguard Breastplate
Glorious Breastplate
Lofty Breastplate
Verrrrry hot, isn't it? Expect to sell these for some good gold too. They don't sell as well as cloth, because plate has a smaller market. But with the rise in DK population in Silvermoon and Stormwind RP sections, you're likely to find a buyer for 100g.
LAST TIP:
If you're not a girl IRL, like me, don't try to bet on other items that you think that "look nice and worth more". The list of stuff which are sought after by RPers is rather short.
The best way to scan the AH for RP bargains is just go for the cloth > chest section on the AH rather than type all the names I mentioned here manually.
Good hunting y'all.
I can assure you it works. I've a lvl 1 banker on a high-pop RP server which started with 0 gold and reached 1088g within 2 weeks, just by selling vendor-shirts and reselling RP outfits.