well this is another guide about making some "easy" adena
i like to call this method "playing the market" because its actually a game, where only rule is:
-BUY CHEAP, SELL EXPENSIVE

1) Ancient adena..
seeing how AA prices are going up these days can either;

*Buy AA from everyone selling at a "Low" Price and re-sell at the actual price everyone is selling your profit can go from 1 to up to 4 adena depending on server.
*Buy Seal stones from people, usually selling at a reasonable price, cheaper than AA sometimes. Price of Seal stone / # of AA per seal stone = Price of investment, usually can win from .8 (yea sounds like a joke) up to 3.8, though this option is the more easy to do.

2) Materials.
sometimes they look like cheap stuff or unwanted stuff, but for dwarves they are relics.
Varnish, Stem, Coal, Charcoal, Cokes, Thread, Silver nugget, Mithril nugget, Stone of purity, Animal bone, Coarse bone powder, usually some people dont even bother to pick them up when leveling, so what can do is set store 1 or 2 nights looking to buy them in any "low level" town like dion and then can go to sell what you bought cheap to giran, where people wanting to make items or to have made are willing to pay a nice amount of money for them.
Usually profit from 100 adena up to 5k depending on item and offer (remember to play with offer and demand, some people when most desesperate will be willing to pay x2 or x3 the price of some ingredients)

on a side note, having a scavenger dwarf for playing when got few time or when just want to relax/mess up can easily get you a LOT of ingredients, you just wont care about exp, you will be checking if you have got 1k or maybe 2k of a same item :P

3) Re-Selling Seal Stones.
this is similar to point 1 but also works, sometimes may find cheapseal stones somewhere while other people sell them 1-3 adena more expensive and it works too if dont want to bother exchanging to AA or just cant.

4)Recipes.
this isnt exactly buy and sell, but there are some recipes that most people are looking for and some are just very easy to farm, takes a dwarf scavenger leveling 10-20 to get them, stuff like varnish of purity, Soul shots, blessed spirit shots (will need around 30 for this one) may find people paying 50k for a recipe that you just spend 5 min - 10 to get

5) Scrolls
if talking about Re-Selling this would be a very good one, sometimes you may find people trying to sell enchant scrolls or blessed scrolls in a way cheap, just check price everyone offering and buy the cheap ones if they got few (remember if someone is selling cheaper a LOT of the same item, everyone will buy from then and you just wont profit) buy them all and then just re-sell.
i have seen that can get 50k - 700k on this way (blessed scrolls of resurrection got the big money)

6)Enchanted Weapons
*WARNING* try to not OVER ENCHANT or this wont make you any adena, you will actually LOSE if you fail.
i suggest this one ONLY if its a weapon you will be using long time and dont matter if you break and can just try again, and when done got to re-sell.
Buy a D-Grade Weapon, then buy the number of enchant weapons you plan to use, enchant your weapon, use and when you're done with it try to sell, sometimes when its past +4can be sold for double or triple its non-enchanted price
for example.... i have seen glaive from 3.5M and each enchant at 300k, so if buy 5 will be 1.5M on enchants, let's say you get lucky and dont break any, then you bought a +5 glaive for 5M but you can sell back at around 7M

just a few suggestions for making some easy adena, remember that have to take your time and be patient, can get some nice adena in long-time term